Squeaky something after chain falling off...
On today's ride my chain half-derailed from the sprocket, freezing it into place (quite scary on a 25+kmh downhill,,,) I of course had the tools, and fixed it but after that the whole thing started buzz-buzz-buzzing when I start or go uphill and squeak-squeak-squeak-SQUAWK-SQUAWking when I start to ride faster, at speeds above 22 km/h squeaks nearly disappear. Chain tension seems to be OK (I can move the chain 1.5 or so centimeters up and down)... What could it be?
Single-speed, coaster hub, 230-something km total on the cyclocomp, rode 15 km more with that squeak.
Derailed single-speed? Check the chain for bent/twisted links. You can bend them back with a couple of pliers, or just replace the chain.
Might also want to check your chainline to make sure it's straight, and chain tension to make sure it's snug.
Found the squeaky part. Not drivetrain-related at all. It was bent kickstand, rubbing just the edge of tire's tread, no (major) tire damage. Don't know how I managed to bend it like that.