More importantly, around what RPM range does this happen?

Is it more common on the lower gears?

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the rpm range varies, but its usually around the lower rpms in the lower gears, mainly 2nd. my friends say that it sounds like its comin from the middle/rear part of the car, and not the front.

i just got an oil change and the oil level is normal.

 

Bringing this back from the dead...I'm experiencing the same exact issue. 2003 Sedan 6MT. G-mented, has flushing the cooling system fixed this issue for you?

 

Yeah, I know this is a dead thread, but I figured I'd toss my two cents in. I have an 03 6MT coupe and I get the same thing every 6-8 months. For me it sounds like a slight rattle around 5500rpm - 6500rpm under hard load. I've found that it usually signals it's time to run SeaFoam through the fuel/intake system. That clears it right up for me. Not sure the cause, but that seems for fix it for a while.