Ok for the last 3,000 miles or so I've had a tap in the top end on my KLX250. Its only there from 5,000-6,000 RPMs. It is most noticeable at 5500 RPMs. When this intially happened I checked the valve clearance and the timing chain tension both are good. I then checked the ACR spring. It seems a bit weak IMO. Does that spring wear out at 2,000 miles?

Does anyone else have a similar noise in that RPM range?

 

not even gonna get into it.... but im guessing its the acr... mine had a noise at just under 5000 rpms....

 

I doubt it could be the ACR. The mechanism is simple. You might want to check the timing chain. I have a similar issue. Just be adding tension to thetiming chain makes the noise go away.