What would make a whining noise at 3500 RPM but only when rolling (in gear or not)

So I'm still trying to figure out this whining airplane like noise that occurs from 3000-3500rpm and again at 5000+ rpm. The issue is, it does not occur if I'm standing still. The car has to be moving at about 25mph + and it doesn't matter if the car is in gear or not. If I accelerate I hear the noise. If I push in the clutch and rev I hear the noise. If I come to a stop and rev, I do NOT hear the noise. It increases in frequency when revving. It seems to not be as obvious between 3800-4800 rpm. It's slowly driving me mad. I hear it easily with to top down. I did replace the alternator and pretty sure it started after that and so it could be from that. I bought a brand new, 997 version, Bosch. I have one idle pully bearing that spins freely (which I've heard is sign of bad bearing). It is the one attached to the alternator. But wouldn't an idle pulley bearing make a noise no matter if the car was moving or not? I have found an odd low resonating sound when the AC is turned on. I'm just adding that in as it's part of the belt system, but I don't run the AC very often so not sure it's related. I have a stethoscope and have tried poking around while idling and manually revving engine ( cable actuation ), but I can't get the same whirrrr to occur. I'm running out of sanity at this point. I might put back in my bad alternator and see if that changes anything, but it was overloading and my volts were spiking so hesitant to do that. Does the transmission rotate at all when the wheels are in motion if the trans is in neutral? Any part somehow connected down there?

For putting back the replaced alternator you need to remove the belt. You then might as well take the car for a drive and check whether the noise has gone. Not worth putting the replaced alternator back in, if it hasn't. In neutral and rear wheels turning the pinion shaft and the gears that are fixed to it, rotate too. These turn the gears that are meshed with them on the other shaft (primary shaft, connected to clutch/crankshaft). In neutral these gears will turn (driven by those on the pinion shaft) but are not connected to the primary shaft. The primary shaft will turn at engine rpm if the clutch is closed as then it's connected to the crank shaft (irrespective of transmission in or out of gear). With clutch depressed (rear wheels turning AND transmission in OR out of gear) the primary shaft would theoretically not turn but drag in the needle bearings between turning primary gears (driven by gears on turning pinion shaft) and primary shaft itself, will drag the primary shaft along so it will turn slowly. Pinion bearing (#2 in diagram linked below) is known to make a whining noise. Search to find out if it is under the conditions you describe. It does sound like it as it is always turning under those conditions and its rpm is defined solely by vehicle speed. http://www.autoatlanta.com/porsche-parts/hardparts.php?dir=996-99-05&section=303-07 Saw in your other thread you've got +100k on your car/gearbox. Very probable your pinion gear bearing is shot. Allow me to say that you need to pull your finger out and remove the belt considering that was suggested in the other thread over 6 weeks ago

Quotes source post: 19104882

I did remove the belt and rev at stationary, but I wasn't aware I could actually drive the car with it removed. I would be too worried of high heat, especially in socal summer (yeh we're in fall now). Is it safe to do so? How fast do the temps climb with no water pump running? Thanks for write up on trans. I do remember reading about pinion and similar issues, but I think it was ruled out because of it occurring in or out of neutral. If it's safe to drive without the belt I'll try it, maybe early one morning.

I would not drive the car with the belt removed. What I would do is get up to speed on a straight piece of road then put in the clutch and turn the engine off then turn the key back on to keep the car powered up. Just a quick off-on. DO NOT pull the key out as that locks the steering column! Coast with the clutch pushed, trans in gear, and the engine stopped for a bit and see what happens with the noise. The put the trans in neutral with the clutch still pushed and listen again. When you want to restart the engine put the trans into an appropriate gear and slip the clutch to restart the engine.

Quotes source post: 19105025

Good to know you had the belt off and engine running at 3500 rpm. If there was no noise that result isn't conclusive unfortunately as the pinion bearing wasn't rotating either, so it could still be that. A damaged bearing will make a noise when it's loaded, if unloaded not so much, i.e when coasting with clutch depressed or transmission in neutral. You'd have to run the engine from cold to avoid overheating when you take off the belt and work quickly. I understand the reluctance to do this test, however. A shorter belt that by-passes the generator would be a good way to rule it out, not sure if practically possible.

Quotes source post: 19104562

Yes. If the vehicle is moving, the trans is moving, even in neutral, even with the clutch depressed, and even if the engine is completely off. If you haven�t replaced the fluid in your trans, and you haven�t checked it in awhile, you might want to go and replace it. I use Porsche only trans oil in my trans, even though there are other brands out there. It�s not cheap, maybe close to $200 all in just for the oil, but it�s easy to do yourself as long as you do this while the car is completely level.

Quotes source post: 19105025

I would only drive the car for less than 1 minute if you take the belt off. As others pointed out, a cold or warm engine is fine, but not an engine that is fully up to temp.

IMO, Do not drive the car without the belt. Our rather non durable delicate all aluminum engines that crack heads from a little piece of plastic blocking partial flow in just 1 coolant passage, don't need the water pump not running in all coolant passages for even a moment while combustion heat is generated.

So given the transmission (parts of it) does rotate while in neutral and coasting, is any part of it affected, pinion bearing included, if coasting while in neutral, and revving the engine. This is the part that has me befuddled. Neutral, coasting, and revving makes the same sound as in gear and revving at exact same RPMs. Stationary, neutral and revving. No sound. Appologize if in replies above this is clearly a yes, but I might need it clearer

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I did check trans fluid a few months ago (had another thread where had to heat the check level bolt up to get it off...). It was over filled and I merely let fluid drain out till it stopped. Fluid color looked pretty good.

Quotes source post: 19105065

Well The problem is the noise doesn't happen unless I rev the engine. I can coast at 70 miles an hour and hear nothing but if I coast at 70 miles an hour and rev the engine to 3500 I hear the noise so coasting with the engine off it's not likely to produce any noise at all either.

have you installed an amp..?

Based on your prior audio clip, it�s not belt or pulley whine and it�s not behaving in a way that would suggest that. You need to nail down source. I think you should get a $20 Bluetooth mic off Amazon and some gorilla tape and record audio clips from various points in your engine bay and wheel arches. Whatever it is, it�s weird and unlikely to be the usual culprits. Or - if you have or can borrow an iPhone and AirPod headphones, there�s a feature called �live listen� that allows you to use the phone as a remote mic. Gonna have to get creative like that.