sounds like a vakum leak to me, did you check rubber hoses and that gasket inder the plastik carburetor thing?
Quote Originally Posted by MBXB
Don't know if this makes a difference or not, but another detail
I forgot to mention is that this sound only occurs after I've run a few miles, when the engine is well into operating range, and over 45 mph.

After that, I can replicate it parked or moving.