Low oil pressure indicater light comes on after car warms up. Light turns off if rpms are higher than 2500

Either fault in wireing , or dodgey oil light , needs a litte Diagnostics

Low pressure when at idle, after the engine has warmed up and the oil has thinned, indicates that the oil pump is worn and needs to be replaced.

Have you checked oil level?

You oil light does not only show when your oil is low, it also indicates when your oil pressure is low. If the light is staying on all the time, even when at a level, higher rpm, then you most likely have a problem with your pressure and the issue needs to be checked pretty soon. If it is only doing it when your car is idling low, then you just need to take care of your electrical/gas or air intake problem first. The oil light will come on if your car is idling low, but it should go off when you get to speed. Does it go off?

Your motor oil level is low.
The oil light indicates low oil pressure - at higher RPMs the oil sending unit (oil pump) can create
a little oil pressure, but at lower RPMs it fails to keep up.
If the oil level is good, then the oil sending unit itself is going bad.

I think you should have an accurate mechanical oil pressure test gauge installed in place of the oil sending unit and check to see what the actual oil pressure is. If it's 15 psi or so at an idle and about 35 psi or higher at 2500 rpm then your pressure is good and means you have some sort of issue between the sending unit and gauge. If the pressure is low, say below 10 or 5 psi at an idle with a warm engine and doesn't improve much at higher speeds then you actually do have low oil pressure, most likely due to worn bearings inside the engine. An accurate test gauge should tell the tale and help you figure out what is wrong. Hope this helps out a bit!

It's most likely one of two things: Your engine could be low enough on oil pressure when idling that the light is coming on for that exact reason or your oil pressure sensing switch could be faulty and misreading the actual oil pressure and erroneously turning on the light. The only real way to tell is to have someone remove the oil pressure sensing switch and temporarily install a mechanical test gauge in its place to check the actual oil pressure available. If it's low then there is an internal engine problem of some sort. If the pressure is about 10 psi or higher at an idle and 30 psi plus at 2500 rpm then your sender is at fault.

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,

your oil sensor comes on at less than ten psi, you may have internal engine damage how many miles on engine, any gum up inside engine, you can try to put on a high pressure high volume melling oil pump to try to help for the time being, but you may need to have it torn and repaired.

have the oil pressure checked with a mechanical oil pressure test gauge to verify the true oil pressure, this could just be a oil pressure gauge sending unit problem, this is very common.