1999 cavalier z24 problems

I have a 99 cavalier z24 5spd, it has 259200 km's on her but drives great....in the last week or two my serpentine belt has started to make noise (i need a one) but now my engine is knocking, i think its a rod on cylinder 3 becuz my check engine light keeps coming on saying cylinder 3 misfire. i put new plugs in her at 253000 (ac delco platinums) any1 have any ideas why its knocking......i know its not the oil becuz i just changed it 500 km ago, and i'm putting a oil pressure gauge in to see if i'm getting proper pressure

some help would be great

Those engines are known for spinning rod bearings, so it's very possible. If it knocks on accel and decel, then it's probably an internal problem... only on accel could be preignition, does it do it both hot and cold?

ya it does hot or cold, and both on acceleration and deceleration

Sounds like it's probably time for an engine rebuild. Using a high performance oil pump will help prevent another failure, pretty common on these engines.

how much would it cost for a engine rebuild, becuz the car is immaculate, and i'm thinking of just finding a Z24 motor outta a 2000-2002 z24 4dr, think it would work just to drop it in and plug and play on the ECU from the car i buy the motor from ?

Call around for the cost of a rebuild.

If using a salvaged engine, they dropped the EGR valve late in '99, so you may have to swap the valve for your block-off plate. They also changed the impedance of the injectors sometime around 99 (not sure of the year there). It's usually best to use your known good injectors, but if you just want it to be easier, be sure to check that they're the same before you drop it in. I'd suggest rebuilding any engine anyway, and especially one of these since the oiling problem was never fixed while they were in production.