Another engine problem

I have a 1997 Pontiac Transport with a 3.4L engine, approx 150k miles, engine replaced about 10,000 miles ago, 2d engine from junkyard - mechanic replaced upper and lower intake manifold gaskets. Vehicle was idle for about 3 weeks, went to start, made a terrific rattling noise, oil pooled on garage floor, did not leak before, van appears to be running fine, haven't notice anymore oil spills. Possible rod bearings? Any advice?

thanks.

I'd check the engine visually top & bottom for anything obviously wrong. Might be very evident where the oil leak started. I'd also check all fluid levels to see where that oil came from. Might be from the tranny or p/w steering pump systems.

Something may have let loose & cut thorough a line running some fluid. Some fluid level has to be low.

As I understand, after the rattling noise & loss of fluid, engine runs fine meaning no unusual noises & oil pressure is ok?

Mechanic checked it out and did not notice any damage, said the oil leak was due to faulty drain plug washer. Changed the oil, runs fine, also replaced the rear o2 sensor, service engine soon light is now off! The one I used to replace before (Borg-Warner brand from Ebay) must have been faulty, replaced it with genuine AC Delco o2 sensor - $100 - ouch! Well, at least the engine's running good, but now the trainy's starting to slip.

Good to hear ..about the motor. What part# is the O2 sensor? Looks like its not the same as the front.