Mistery clanking noise?

We just bought a used 1992 Oldsmobile silhouette,we test drove it and it was fine, but on the way home with the van my fiance turned the air conditioner on and the van started making a clanking noise that has not stoped. we have located roughly where the noise is coming from but confused as to what could be making the rackett. The sound seems to be coming from behind the passinger side tire. posably from the main drive pulley or a/c compressor.
And when the van is idleing if we turn the air conditioner on the van will shut off. Can any one help us please?

Sounds like the compressor is going bad. Try taking the belt off of the compressor and see if it goes away. Just make sure the belt doesn't drive something important, like the water pump. If you have a serpentine, try buying an A/C delete belt.