Engine Rattles when AC Engages

1997 TP, with 155K, 3.4 L, on its second engine from junk yard, the van runs great, mileage isn't anything to write home about, 19 mpg. When I engage the AC, a rattling noise is heard coming from the engine well, (the unit cools ok, could be colder), I believe the noise is coming from the compressor.