I had the very same problem about 8 years ago. A mechanic who is no longer in the area said he "cleaned" the throttle, and it was good for 8 years, but now it's started again. The mechanic has left, and I'm not sure what he cleaned. Adjusting the air screw can minimize the noise at idle, but then it just returns with a different pitch with the throttle cracked open on light accelleration.