First, welcome to the GWOF from Pennsylvania. I'm not a wrench guy, but let me ask some questions which may help. I assume the sound is there when you rev the engine at a standstill in neutral and goes faster and slower as you increase and decrease the throttle. If that's so, then it sure sounds like a belt problem. Do you know when the belt was changed? Could it be stretched? Don't know, but is there a tension mechanism that could be failing..