I have a rattle type knocking in the engine when the engine starts or hits a certain rpm around 2500

if on a V 6 petrol @ 1500 rpm
could be the knock sensor
distributor timing might be out
or serp belt tensioner bearings

if a diesel i would expect a rattle@1500 rpm

You should be able to use a long piece of rubber hose to listen around the engine and find the source of the noise. I am assuming you have checked oil pressure.

sometimes if you use 87 in your car it rattle try a higher one and if it still rattles could be the valves

Those model lifters are quite notorious for clogging up with hardened oil as they ran a bit hot... my suggestion would be to service all your lifters or replace them up to you and make sure all clearances are correct at tappets and valves :)
Good Luck :)

Is the oil level ok? I assume the car is still under warranty? If it is, don't drive it, as damage to the engine may result. Call the dealer and explain the problem and have it towed to them. If you drive it, they may try to get out of a warranty repair, saying you 'abused' the vehicle by driving it in this condition.

RPM and speed do not match what I'd expect to see at 120. but that could be gear ratio being different than I'd expect. Most don't like to run at 4k. Most problems at that rpm is either plugs (experiment with different gap settings or change heat range one step colder or hotter) or can be coil related (work fine except when recovery time is shortened due to rpm). However, the weird noise may be due to a partially plugged catalytic converter. So, check that as well. If you have a knock sensor, test that too.

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,

I had the same problem on my 2006 Lucerne and it was the engine mount located on the front of the vehicle behind the radiator.

Have you checked the catalytic converter (also reffered to as simply a Cat by mecanics)? It's the first big canister you encounter on the exhaust line right after the manifold and before the resonator and/or muffler. After a while rust and time can cause either the casing itself to come loose and rattle, or the insides themselves break up and rattle around the cat. And being where it is it often resonates sound around the engine compartment, making it seem the sound is coming from elsewhere than it actually is. And while your at it don't forget to check the flex pipe usually connected to the cat also (some models do not have a flex pipe though). If all that is fine, then the other things to check would be the accessory brackets, wire holders and the like. With vibration and time, sometimes they loosen up and start to rattle. Still nothing? check the suspension parts for anything loose (IE. sway bar links, coils springs, tie-rod, suspension tables). Other than that there isn't much in there that can rattle. If you look at how an engine is built, you'll see that there isn't much inside that can rattle around without causing the engine to fail altogether. I'm not saying it can't happen though, but it's highly improbable. Hope this helps

Sounds like the timing belt tensioner. I have that problem. Rattles at low rpm, goes away as you acceralerate. Looking to have timing belts changed and tensioners replaced.