Subtle vibration at idle? (amongst all the other ones lol)

I read through a lot of IMSB posts here last night but still cant quite place what I'm feeling subtly in the pedals or steering wheel at idle. Symptoms: Only felt at idle (idle's a little rough but not audibly, tach slowly bounces a little at 6-700ish rpm when warm). The vibration or whatever it is isnt perceivable when moving or above idle. It feels as if there's "something" slowly rotating while at idle and there's some friction during part of the rotation that creates this very very subtle vibration that can be felt in the brake, gas, and clutch pedals (I feel it most in the clutch pedal while at a red light). It's very cyclical. Repeating about 2x every second - 120rpm. What turns at that rate? IMS? It's felt regardless of being in or out of gear. Any ideas? Went through a tbelt failure on an Audi and really NOT into doing a 996 head rebuild (+ more from what most cases sound like). Thanks for any words of wisdom.

My first thought is worn motor mounts. (and possibly transmission mount) more info http://www.pelicanparts.com/techarti...eplacement.htm

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As frisbee91 offered there are the motor mounts. However, the behavior doesn't read like motor mounts, but it doesn't hurt to check them, or have them checked. The car has be safely in the air and the tech should know how to check for failed motor mounts. Are there any pending diagnostic trouble codes (DTCs)? You'll need an OBD2 code reader/data viewer to pull these codes. Absent any more to go on other than what you have provided my WAG is the AOS is going bad. Rough idle. Variable idle RPM. With the engine idling you can unscrew and try to remove the oil filler tube cap. Since the engine is not misbehaving that bad (yet) I believe you'll have no trouble removing the cap against the low pressure in the crankcase, but give it a try, if you want. Even a healthy engine will react when you remove the cap -- but should adapt and recover within a couple of moments after you remove the cap -- so do not be concerned about this. The engine will react and recover when you install the cap which is of course also normal. Before you remove the cap check for any vacuum leak at the cap. With the cap securely tightened try to move it about on the filler tube. Do not use too much force but see if by moving the cap about you can create a vacuum leak. You can hear this and the engine should react. It is not unknown for this cap to leak (old age). Always though when tinkering with the engine in trying to make some kind of diagnosis of the engine's abnormal behavior keep in mind if the engine gets worse, shut off the engine and get the car flat bedded to a qualified shop and get the behavior diagnosed and fixed. There are other things you can do if you want to try to diagnose this yourself. One thing I did when trying to figure out what was going on with my car's first AOS was to remove the throttle body. I found it oily and the butterfly valve wet with oil. I cleaned it and carefully reinstalled it. The engine was better for a while -- not very long though. What I also noted but at the time its significance didn't occur to me was there was a lot of oil at where the hose/fitting from the AOS to the intake manifold was. This is located right behind -- downstream (closer to the engine) -- of the throttle body and was a sign the AOS was passing through way too much oil vapor.

I played around in the car a bit more today. I think motor/trans mounts absolutely need to be looked at as some shifts are met with a pretty horrible *thunk* that feels like an elephant fell out the back. Will take a look with oil cap tomorrow. Thank you both for your input.

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The effects of bad motor or transmission mounts vary. Shifting difficulty (as well as abnormal vibration) can arise from bad mounts so you definitely need to have all these checked. But be sure the brake and *clutch* fluid has been flushed and bled recently. My experience is if this is overdue (in my direct experience I let it go 2.5 years while it should be done every 2 years) the clutch behavior degrades and shifting becomes a problem even to the point that the box crunches some going from gear to gear.