Scooter runs fine but makes odd noise it didn't make before?

I have a 150cc, 4 stroke scooter. Not sure of the make or model.

I was using it regularly for a while, and it worked fine, but then I was pulled over for not having a motorcycle license (go figure) and told not to drive it until I got one.

So it's been sitting in my driveway for a month...I've done my best to protect it from the elements, and every few days I've been starting it up and letting it idle for a few minutes.

Today I took it out for a quick ride around the neighborhood just to make sure it was running ok, and it ran fine, but now it makes a noise it didn't make before.

A few details: The noise is hard to put my finger on...it's best described as being somewhere between a rattle and a vibration. It's almost like the engine is wheezing. And I can sort of feel it in the seat, too...it comes from the engine. And this ONLY happens AFTER I get it up to a decent speed (20-30+) and THEN let off on the throttle. The sound dies once I've stopped.

Like I said, the thing still runs fine, I just want to fix this problem before it becomes serious...and I also don't want to spend $225 on motorcycle courses just for the scooter to crap out a week later.

Any ideas?

I should also mention I don't know much about engines in general, so any answers should be in layman's terms, please.