My rpm drops to 500 when in idle mode after its warmed up and then oil light comes on

Clean the throttle body inside and the idle air control motor.This will help.

You might get lucky and have a faulty oil pressure switch.More likely though,the oil pressure is dropping at idle due to a worn engine.Replace the switch and hope for the best.If the trouble persists,you'll need to do engine work

Jeff did you figure this out. I am dealing with the exact same thing currently. Things to look at are Mass Air Flow Sensor (MAF), Throttle position Sensor (TPS), Air Idle control valve. I have played with everything but the Air control valve. I tried three different MAF, including a ghost sensor (circuit to match ressitance). I'm going to investigate the air idle control tommorrow. Let me know if this helps or if you get it figured oot

Does the oil light come on or the knock occur during the engine warm up after a cold start?
There could be a couple of problems causing the issue. You could have an oil pressure pump or pump pressure regulator sticking. I would have a mechanical oil pressure gauge installed and verify oil pressure at idle and 2500 rpms to make sure oil pressure is in normal ranges.
You could also have excessive piston rod bearing, crankshaft bearing or cam shaft bearing wear that will cause oil pressure to drop too low to prevent the rod knock your hearing at 2500 rpm.
You said a knock and not a ticking noise,so I don't believe a lifter is the issue at 2500 rpm.
Start with the oil pressure check and go from there.
Let me know.
Regards,

sounds like you have to adjust your idle, fuel injected or carburated?

could be a vacume leak. check hose for cracks or lose conections.

with a 2002 Concord 3.5 V-6, what should the rpm be at 60mph? 70 mph?

I'm not sure if the overdrive gear is lost in the tranny, but the transmission light is on.

Thanks

how low does the oil pressure drop?? is it below 40?

Sounds more like the Throttle Position Sensor or another sensor is letting the engine RPM's drop too low. When the RPM's are too low, the oil pump is not pumping enough oil.

got same prob help us !!!