Odd engine sound - any ideas?

This just started - it had happened once before after the pitot tube got bumped, but went away almost immediately. That was six months ago. I had the spinner off last week for annual and now the sound is back, and louder than before. It's just in the one RPM range you can hear in the video.

YouTube link:

https://youtu.be/6INcpq_3CnY

Update: talked to my Rotax guy, he's thinking gearbox. I'll have it looked at tomorrow.