My wife's Scrambler 400 2wd has begaun to make some noise while under load. I can only describe the noise as a low frequency "thump" that I think is coming from the belt drive or the gearbox. At first I though that it was the sign the bike needed a new belt. I have changed the belt, but there has been no improvement. My next thought was to pull apart the primary clutch and see if anything looks wrong.

Any suggestions would be helpful.

 

It could be the drive chain, or maybe a tooth missing on one of the sprockets. On high mileage machines, there is a silent chain in the transmission that can eventually stretch and give a clunk noise when under load. Let us know more.

Waylan