My Downtown does this. On take off, it makes a vvvvvttt, vvvvvttt, vvvvvt sound like I'm holding in a clutch and revving the engine. And we know there's no clutch. It doesn't always do this either.
I do use seafoam and that seems to be better than if I don't add it, but I think I'm going to try high-test gas on the next fillup to see if that helps. The dealer told me I could use regular, but that may have been bad advice.