I would definitely go take a look at the bike. If the flywheel noise isnt something that sounds very obviously wrong then pass on it.
The sound could be the oil pump which it most likely has. Those things can go bad and it might be on it's way out. It is usually a good idea to delete those anyway and just run premix in your tank rather than let the pump do the mixing. The pumps are powered by the spinning of the flywheel, which is why I mention it.
If it seems to be running besides that noise and everything looks to be complete on it too then scoop it up. We can help you repair any issues. Just go with your gut and dont be afraid to walk away. If it is anywhere around $500 or less it is probably a pretty good deal.