Hi and thank you for your question.
I am happy to help you but need a bit more information to be able to provide top quality service and accurate information for you.
1. Exactly what model 1989 2 stroke Yamaha do you have?
2. Also, when do you hear the noise? At start up, after warm up, all the time, only when accelerating, on decel?
3. Is this something new that has just started happening or has it been going on for awhile?
4. Is the bike new to you or have you owned it for a long time?
5. Has the bike been sitting for awhile unused and if so how long?
6. Has any service or repair work of ANY kind been done to the bike recently, especially around the time the noise showed up?
7. Does it seem to be coming from the engine?
8. Can you hear it while the bike is running but sitting still or do you have to be moving to hear it?
9. If you are riding down the road and pull in the clutch to coast, do you still hear it?