Got a rattle from what sounds like under the car when the air is on up middle to floored throttle. Only happens when the air is on. No loss of power, no trans slip nothing. Only happens when the air is on. Thoughts?

 

Does it only happen when the air is on? Just Kidding, its probably a worn out Compressor.

Edit: O yea or a bad belt tensioner....

 

Yea I was thinking the Belt tensioner too. I'll have to check it out today. I get alittle belt slippage when coming off full throttle. Might have fubared it when I got the cam swap done. I'll check it out. THanks for the insight.

 

That would be the easiest thing to check. Next could simply be a worn out belt. But I've in the past had the problem with extra load on the accessories from the Compressor bearings being shot. Eventually if this is the problem, it will seize. Hopefully in the off position One other thing to look for could be a dirty IAC. When the A/C flips on it should rev up a little. If not revving up high enough then this could be the problem. I'm assuming you have an A4. A lot of A4s have idling issues with the A/C on. What are your RPMs at when you have it on versus off?