My 2004 LeSabre has developed a problem that also occured on my previous 2000 LeSabre in its old age. Unfortunately after numerous trips to the dealer, the problem was never solved on either car. With the A/C turned on under hot driving conditions after the engine has come up to temperature the problem will occur frequently. Essentially when I accelerate I can hear a loud grinding sound under the hood on the passenger side. The noise will abruptly cut in and out as the engine revs up. You hear it in pulses as the engine revs up to redline. It is especially loud under very heavy acceration from a dead stop. When the A/C is switched off all of the noise goes away and I hear nothing unusual. The dealer installed a brand new compressor and it didn't solve the problem. It almost sounds as though two pieces of metal are vibrating together. Although the dealer insists the problem would have to be the compressor, I tried to argue that the normal vibration caused by the compressor working could be triggering some other component in the loop to vibrate. Unfortunately I can't replicate the problem with the car standing still. It simply doesn't make the noise unless the engine is under heavy load at high rpm's. I would describe the noise as something that sounds like a coffee grinder.