My wife's 2005 A4 1.8T auto is having an odd rattle/vibration when the air is on but only at certain times.

When the car is in park and the air off, there is no rattle/vibration

When the car is in park with the air ON, there is no rattle/vibration

When the car is in drive or reverse and the air off, there is no rattle/vibration

Edit: When the car is in drive or reverse (not driving, just sitting) and the air is ON, the rattle/vibration is there. When you start accelerating and driving, it goes away.

Since it only happens when the air is on and the car in drive and no other time, it makes me think that it has nothing to do with the AC.

Maybe the tensioner or motor mounts? Any thoughts?

 

Does it sound like metal on metal?

 

Not really a metal on metal sound and it's not a rattle like when a cat is going bad. It's kinda hard to explain so I'm going to try to get a video of it tonight when I get off work.

It sounds like something is loose and vibrating which is causing the rattle, if that make sense which got me thinking it could be a tensioner that is wearing out and when the extra load of the AC is put on it, causes it to rattle.

It doesn't sound like a pulley bearing is going out on anything either. It's just strange because it doesn't do it when the air is on and in park, only idling in drive/reserve. It goes away when you start accelerating and driving (didn't make the clear in my original post).

 

I had a rattle very similar to what you're describing on my 2.7T and it was a heat shield that needed to be bent away.