I have some kind of issue in sixth gear only that it seems to grind or make a grinding type of noise at highway speed.  I can drop it to fifth, but with a 15t and fifth gear on the highway, it can be pretty rough.  What would cause an abnormal noise in sixth gear only.  I have searched the topic and can find nothing with our lovely search system.

At what RPM?

Is this something that has just started? or is there a history, revs and speed might give some clues.

Always eliminate the simple and obvious before looking for the complex technical reason, check oil level and chain tension & condition.

im not sure if this is what you mean, but ive always had a different tone when using 6th gear then on the other gears. its always done that so i dont really pay attention to it, plus its how u know your in 6th gear 

If you are getting a grinding or metal-on-metal sound in top I would put my money on the sixth gear set being worn. The other possibilty could be a failed layshaft or mainshaft bearing but if this was the case I would expect the noise to be there, or be starting to develop in all gears. Either way it will only get worse and it means major surgery to rectify, these issues cant be rectified from the outside. Bike gearboxes are all the same when they fail, if the shaft bearings dont go first its either 1st wears out due to the torque it has to transmit to get the bike moving or top wears out because it gets the most use.
I would be starting to look for a decent 2nd hand engine unless you have the time/resources/confidence/desire to be tackling a complete engine strip-down and gearbox rebuild.

It seems to sound like a metal on metal sound.  I checked oil (first of course), chain tension is perfect, alignment is great, the only thing that has been changed lately are the sprockets.  My old ones never seemed to make that sound, the new ones do.  I don't understand only one gear though. 

Quote from: CndnMax on August 15, 2007, 10:10:27 AM
im not sure if this is what you mean, but ive always had a different tone when using 6th gear then on the other gears. its always done that so i dont really pay attention to it, plus its how u know your in 6th gear 

yep same    6th gear sounds different to all the rest.... not sure I'd describe it as metal on metal though as the original poster mentioned...