Strange tick from "differential" - coasting and in gear

Hi, I have a 1999 C2 manual with 100k miles. The last few weeks a tick has developed from the differential. It's a very light "clonk", it sounds like the tick on a prize wheel. Ti-ti-ti-ti-ti-ti. It feels like the noise is generated once per tire revolution Sound appears: - Increase in frequency with wheel speed - Appears from 20 mph up to 60mph (probably still there but hard to hear) - Gear or natural doesn't matter - Engine on or off doesn't matter - Sound is there with rear on jackstands and wheel spinning in air (with engine on and in gear) - Turns or straight doesn't matter I have been under the car while it's been in gear and the sound is coming from the "diff" part of the transmission. There is nothing touching on the outside. Any clues? What would be the next step?

Sounds like a nail or something in Tire. Check there first.

Playing card in the spokes? Loud enough for you to hear from the cabin at 60 MPH? Seems like a chipped ring or pinion tooth wouldn't be that loud. Possibly a CV joint on one of the axle half shafts? Could you remove them one at a time, and rotate the wheel that is still on to see if the sound goes away? This would eliminate them as a source.

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I will not hear it during acceleration under 60mph but maybe maybe when engine is off and coasting a downhill, also have a cab. I have been under the car during 60mph and you can hear it then. (tires at 60mph, car at 0mph. I am not some lunatic crawling under the car while driving..) I will try and remove a half shaft, good idea Never repaired a diff before so a newbie when it comes to this.

Unlikely for many of us to have them, but there is a cool tool called chassis ears that alllows you to clip a mic on various places (like under the car) so you can replicate sounds that only occur while driving.

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I did remove all CV joints so the diff is not driving anything. The noise is still there. I have the car up on jacks and running, I am pretty sure it's from the diff. Anyone knows how to diagnose a bad diff?

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Yea, pop the axle stubs out, remove the side cover and then remove the diff and inspect (bearings and ring gear ect). Can be done with the trans still in the car.

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I got the diff out but all the teeth looks good, no obvious damage. The bearings also looks OK but not sure how a slightly worn bearings looks like, will replace anyway. It does make a metalic noise when I rotate it around, it's not complete quiet. But that could be normal. Do you remember if a good diff should be totally quiet or should you hear everytime there is movement? Will try and record something

Although i have not done a diff in a porsche i have done a couple hundred solid axle and ifs gear swaps. In my experience diff bearings dont tick, they whine or howl. Doesnt hurt to swap them if u have the tools but i would bet your issue is elswhere. Like the cv shaft. Fyi, bearings should feel smooth but can make noise when spun in the race. There is just a thin coat of oil on them.

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I disconnected the CV shafts and ran the car, still the noise. I was hoping that one of the teeth would be worn in the diff but it doesn't look like it. Never seen a diff before so not sure what to expect.

I'm not sure how your holding the diff with it removed and turning, more explanation is needed. Put transmission in neutral, reach up into the diff case and turn the pinion gear. It will have some resistance from the pre-loaded bearings , and 6 freewheel "loose" gears with needle bearings rolling in 75w90 gear lube, but should be smooth and quiet..

Pebble in the tire tread?

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Done the following. 1. Car on jack stands, CV disconnected. The half axle flanges are not connecting to anything. I start the car and put it in 6th. Only thing rotating is the stuff inside the diff, tires are not connected to the engine by any means. Tick sound from diff (I think) 2. Removed the diff from the transmission housing. Put the half axels back in, and rotating it on my bench resting on one half axle and spinning the other. I hear a lot of "clicks" but it could just be the tooth of the gears interacting. Some mechanical noise must be accepted but not sure how much.. (I should get a video) Appreciate the help

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OK.. Doing what you are doing only turns the "spider gears" ( differential pinion gear set). While driving these gears ONLY turn while turning or going in circles to allow different speeds of each wheel. While going straight these gears will not turn. Take close up pictures of differential side bearings and races for inspection. Perform the test I described above on the pinion shaft, It is the most common failure in the G96 transmission.

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Your right! I will do the test you described above.

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The pinion gear moves smoothly, nothing I would be worry about. However I did stick my finger in the drain hole towards the car and my finger was coverted in a dark gray substance. I think this is the grease from the failing bearing... I am a little confused. Is the pinion bearing the bearing that sits close to the pinion gear, close to the draing plug? Or is it the bearing at the other end?

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Both are "pinion shaft bearings", the one closest to the pinion gear is a large dual row angular sealed ball bearing, the one at the other end is a smaller open cylindrical bearing. Pinion just means the smaller gear of a gear "set". You can crank and run the engine and trans through the gears with the differential removed and no harm will happen. The gears and bearing are covered in 75w90 gear lube and will be ok for a few minutes at reasonable speed. Check to see if noise is still there, although it will be at a lesser volume sine it will have less resistance due to the diff removed.

Any progress on this, I have a 944 turbo, from 1986, and I had a ticking noise every once in a while but now its more consistent. I think it is coming from the rear axle, maybe right side, not sure. It seems to occur at 20+ MPH, like the one that started this thread. It seems RPM related but not fast enough to correspond to one tick per wheel revolution and much less frequent than per engine revolution. At 20-40 MPH it might be occuring at "a few times per second". Spinning the wheels by hand with car lifted does not reveal anything. thoughts?

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I will replace the diff carrier bearings and see if that helps. Will let you know once I have them.

Not sure if it helps, but in my 944 which has a combined differential and gear box, that box has already been opened and rebuilt BEFORE the ticking noise first appeared. Not sure if the ticking noise came from some issue in its rebuilding or not. Was rebuilt by a mechanic I trust. Another related item in the 944 is the torque shaft tube and its bearings...those where changed too. That spins at engine RPM however.

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Noise is still there with the diff removed Sounds like it's a transmission issue

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Since you already have the axels and diff removed, it's not much more work to remove the trans. Then remove the rear cover and inspect for chipped/damaged gear teeth. Depending on what you find, you can decide to DIY, or let a pro handle the rest. Some of the gears (Rev, 1st,2nd) can be removed with just an 8inch puller, total disassembly requires a 30-50 ton press.

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Do you mean splitting the case where the arrow is?

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Yep..

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Went back and rotated the shaft driving the diff (output shaft?), it doesn't feel right. Not sure why I didn't catch it the first time. Trans is going out and I will take it from there. Stay tuned! (Thanks for the guidance, much appreciated)

Do you mean the torque tube? There are 3-4 sets of bearings in there. Mine made noise closer to engine is sounded like. Removing them was a pain but solved problem. I put in some "superbearings", cost was about $650, can give you more info if you like.

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The output shaft that is being held by the piniom bearings. whats the torque tube?

ahh, ignore....I got this thread via the 951 forum, and not I see its the 996 forum....you do not have a torque tube. The 951 does...

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Got the cover off. Not sure what to look for but I can't see any obvious issues. The magnet in front of the drain plug is covered in grease or dirt. Everything else is clean (I have replaced tranny fluid 3 times). Is there anything else I can do to diagnose? Everything feels nice. The only thing that look easy to change are the two bearings and the gear in this picture, everything else looks complicated:

Its common for the magnet to be filthy with black ferrous debris, that's its job. Inspect each tooth and bearing surface carefully for imperfections. The two big sealed bearings at the bell housing end will not be visible for inspection.

I sent the transmission out to GBox and they replaced the pinion shaft bearing and 2nd synchro Shifts like new! Cost a little over $2000 including shipping. They sent back the old bearing and it felt a bit "worn" and had some damage to it, very minor if you ask me. If I would do it again: 1. I could probably have waited with the rebuild, I was expecting the transmission to be trash but seemed to be in "good" condition. On the other hand the very subtle noise made me crazy so I didn't enjoy the car. 2. Don't split the case as I did. It's a pain to put together. You can ship it out in parts to GBox, they don't seem to care but don't expect that you can put it back together. 3. If you doing this in your driveway make sure the car is high enough so you can fit your transmission on the transmission jack. I had to wrestle the transmission on to the jack while being under the car and it sucked.