Low pitch chatter when idling - Jetta 04 BEW Auto

Hey everyone. I have an odd sound coming from the engine/transmission on my 04 Jetta BEW Automatic that just arose today, never heard it before. When the car is in park idling there is a low pitch chatter sound coming from the engine/transmission, almost like a low pitch clutch chatter but my car doesn't have a clutch... When I put the car in reverse the thumping gets a bit louder. Neutral sounds the same as Park and in Drive I can't hear it at all. It comes and goes. It has me slightly concerned. It is pretty cold here lately and we have a TON of snow. So maybe there's some snow and ice built up somewhere underneath possibly causing the sound? I'll keep and ear on it and report back.

In the meantime has anyone every heard this on a 04 TDI auto?

I'd be willing to wager your alternator pulley has gone solid (no longer ratchets) and your serpentine belt is shaking like crazy. Same thing happened to my car. Putting a light spray of silicone on the belt would make it go away for a few minutes.

Simple diagnosis: look to see if the serpentine belt tensioner is dancing back and forth at idle. If so, turn the car off, release tension on the belt, pin the tensioner and slip the belt off the alternator pulley. Stick a screwdriver in the alternator fan and see if the pulley rotates freely in one direction but locks up in the other. Dollars to doughnuts, it's locked solid both ways.

Simple fix: replace the alternator pulley with a new one. It would be prudent to replace the belt and tensioner at the same time.

All of this can be done from the topside, no need to get under the car.

I vote alternator pulley as well.

Really appreciate the input guys! I will check out the Alt pulley. My belt is only about a year and a half old so it's in good shape. I did have my alternator removed a couple weeks ago because I thought I was having problems with it, I played with the pulley clutch a bit and it was disengaging as it should but wasn't very smooth, so maybe it was on it's way out? When spinning the pulley the opposite way by hand (disengaging the clutch) should it feel smooth when disengaged? I'll take a look and report back.

Thanks!!

it should feel smooth