If you have a wooden dowel, about the size of a broom stick, or slightly smaller around 18" - 24" long, you can put one end one various area's, and putting your thumb over the other end, then up to your ear, making contact with your thumb, that make's a poor man's stethoscope. Really amplifies sound from a select area. Just be careful not to get it caught in the fan.
I'd try the generator first, then the bracket that holds the fan assy. If nothing there that sounds familiar to what you're hearing, start at the top on the valve cover, and work down, Gov. assy. etc., front to back, and both sides.
I can't remember if this was the thread that had asked about lubing the fan hub assy. or not, but so remember answering to one lately here, or on another forum. If not this one, there is a screw on the round part of the fan assy. It's a round head, and maybe 3/8" across. It has a rubber washer under it. Rotate the fan until the screw is to the side. Remove, then fill with a light weight oil, until it runs out. Let it set a bit, the rotate the fan until the hole is straight down, to drain excess oil. Rotate back to where you can re-install the screw, then try it.
There should also be two small oil fill caps on the generator. Just one pump from an oil can should be enough on those, if they haven't been lubed since you got it.
Between my hearing, and the speakers I have, can't really hear it real well. What I thought I heard though, was something in time on the exhaust stroke through the muffler.
