Does it make the noise while standing still, or only when moving? From your description, it sounds like when standing still, so can't be belt squeak. How may miles on the bike?
Maybe a mouse crawled up into the muffler
I would go over the bike carefully with the engine off and look for loose hardware, especially around where you think the sound is coming from.
Have the head bolts been torqued lately? This needs to be done after the first 600 miles, then less frequently.
The fact that the noise started after sitting for a month is probably a clue, but short of something crawling into the exhaust or intake, I can't think what it might be.