My '05 MCS Cabrio makes an awful rattling sound most of the time when the A/C is on. If I start the car without the A/C on, there is no noise. If I turn the A/C on, it rattles quite loudly, and then even continues but a little quieter if I turn the A/C back off. Usually it stays on until I shut off the engine, and then goes away if I restart. Sometimes, even with the A/C on, the noise will go away after a while. And very rarely, it won't make the noise at all.

I thought it might be the bearing on the A/C pulley, but I replaced that and no luck. Now I'm wondering if it is one of the other pulleys, like the idler, tensioner, or even crank; that only acts up under the load of the A/C. I've been sticking my stethoscope all over the place but can't seem to pinpoint it.

Anybody experienced anything similar? It's driving me crazy, not because the noise is so bad, but because I can't find the source.

 

This is usually a symptom of a bad tensioner dampener. The extra load from the AC makes the tensioner go into an oscillation. Bad bearings usually manifest as a grinding or squealing noise.

That being said, the next time you have the serpentine belt off give all the pulleys a test spin to make sure nothing is binding and try moving the tensioner around to make sure it's properly dampened.

Good luck!

 

You may also just have a warn out belt. Mine made more and more noise until I replaced it. Then... silence.

 

Turns out I was way off. I stalled it driving it up on ramps to change the motor mount, and realized it still rattled with the motor stopped but the key on, for a little while. Pretty sure at this point that it is the PS fan, and I should still be under warranty. The pump seems to be getting noisier too.

Time to go see the dealer.