This could be caused by the Air Conditioner compressor (it is a piston type). Do you get the sound with the A-C turned off?
My '93 Camry has developed a noise which sounds like a rod knock. Does
> anyone have any ideas of what else this could be? This sound is fairly
> new and has not yet presented itself as a failure of the bearing. I
> recently replaced the timing belt in hopes that I might find something
> there. This weekend I will replace the rod bearings if I can get the > oil pan off.
> Jerry