low RPM "rapping"

[turboTony]

If I'm cruising through a trail in say 2nd or 3rd gear with the rpms kinda low(say 1or 2K) and come to a small hill and I get on it, the engine will lug and I hear a "rapping" noise.
Is this normal because the engine likes high revs? It's done it since it was new but if it's not "normal" I'd like it taken care of before warranty's up.
Thanks.

[ZRider400]

mine also makes some weird noise at low RPM. my old CRF450 did the same thing..... i think its normal. These motors are made to be revved high, so at low rpm they sound odd.

[450 Racer R]

An aftermarket exhaust would solve that problem. I think it's something with the restrictive stock one.:confused: Ha I don't know. But that's what I think. Mine stopped that after the high flow exhaust