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.