I'm trying to figure out the source of an annoying rattle on my son's 150. It seems to be the loudest right ofter you let off the throttle. I can't even figure out where it's coming from. Any ideas or has anyone experianced this on your 150?

 

I would start by checking all the engine mount bolts.

 

There is two things I have found that rattled on my 150's. First is the Muffler, the inside baffle pipe fit's loose inside the muffler and it rattles around when you ride. You can feel it and even rotate it by sticking your finger up the tail pipe and moving the part. To solve this I removed the end cap (had to drill out the rivets) then I High Temp siliconed the inside pipe to the end caps.
The second and this is most likely what you are hearing is the chain rubbling in the rear skid plate just under the rear sprocket. The way that is designed and stamped out when you release the throttle the chain drops down and rubs on the skid plate til the power is put back on. I took a jig saw with a metal blade and cut off kind of following the same original lines about an inch from the front of the plate just under the chain. You will see where the chain had been hitting!

 

Thank you!