A/C Compressor noise

[Ali Al-Chalabi]

My variable displacement air conditioning compressor has recently started making a very very loud knocking noise when it is engaged. It is almost louder than the engine but still cools and operates perfectly fine except for the noise. It does not sound like a bearing noise but it sounds almost as if the piston is banging against something at the end of every stroke. If anyone has any ideas what could be wrong with it, please chime in.

Thanks

-Ali