i just got an 06 ralliart with 125k and have driven it less 200 miles since i got it. Yesterday i drive to work without any issues, when i left work it is knocking. No power loss, no smoke, idled smooth but knocks, knock goes away when you rev a little. I have taken off each spark plug coil individually and no difference with either, i took off the serpentine belt to rule out the attachments. I changed the oil and didn't see anything in the oil(no bearing pieces).

What should i do next to troubleshoot?

 

Narrow it down: is knock inside the engine? or from outside/underneath the engine? or a heat shield resonating at certain rpm?

You say its not from a part driven by the accessory belt, so we can rule those out.

Maybe a fastener has loosened and a part is not bolted down securely, or a plastic/metal fastener has broken. What did you last work on?

 

The knock appears to be in the engine, but i can't really tell. The last thing i did was take off the belt. At this point I'm not sure what to do other than tear it down. I was just surprised because no power loss and the noise goes away when i rev up just a little. If the beating is spun around would it effect the compression enough to notice on a compression check?

 

Is it a weak lifter? The hydraulic lifters will make noise with different viscosity oils, try changing your oil. Sometimes your oil will just degrade and the lifter won't hold pressure like it should.

 

I checked the compression and all 4 cylinders were between 190-195. I also put sea foam in it and didn't have any exhaust leaks. I'm thinking the water pump maybe? It's definitely not in the bottom end.

 

Have a look at the crank pulley that drives the serpentine belt. Is it wobbling? Then take it off and check to see if the rubber is coming apart. These things don't live forever.

 

it wouldn't hurt to take the pan off and have a look and the crankshaft, but then it's hard to say without hearing it for myself...