I have a 97 caravan with a 3.0l engine and recently on a trip when we would start from a stop the van would whine like the transmission was bad. Finally after trying to find the sound I figured out if the compressor was turned off it would never make the sound. Turn it on and whine away it would go.

I've had ford's that the piston would knock in the compressor but I've never heard one whine like this. Anyone else ever have this problem?

Thanks
Scott

Two possibilities:

1. The belt driving the compressor is loose. It causes a screeching or squealing noise usually when you rev the engine. The belt should only flex about 1/4-1/2 inch in the middle if proper tension. The underside of the belt (the teeth) should not be rounded or worn - if so replace. If the belt is old, your best best is just to replace it. To try adjusting a loose belt, you loosen the center bolt of the idler arm (this doubles as a lock down nut -15 MM). Turn the adjusting screw above the wheel to tighten, then retighten the center bolt.
2. The belt is the usualy cause of noise. It could also be that the compressor's bearings are worn. In this case, the compressor wheel seems to shake, there's more of a constant metalic noise and the noise will never seem to abate.