Motor noise

I have a 75 hp Merc, (1990) After about a 15 min. run at a little over half throttle it developed a noise ( I could hear while trolling) that sounded like something rubbing on the flywheel or a pulley on a car fan belt. Took in to dealer and the noise was gone. He checked water pump , oil pump and lower unit oil. Ran motor and could not get it to repeat the noise. All seemed O.K. Next day I took it on river and after another run of 10 to 15 min. it started again I shut off motor for about 10 min. When I stated it up the noise was gone. It does not sound like real damage is being done, but I would hate to get stranded on the river. Any ideas? Would a sticking valve be make a noise like that?

Letsgo,
The sounds that you’ve described sounds more like a bearing going out after it heats up a little. Not really sure how you could check other than pulling the top off while the motor is running. Just make sure you don’t put your ear to close to any moving parts. Hope you find the problem, its no fun taking a boat out on the lake when your not confident of its safe return. Good luck to ya.
SEDNERB

Assuming the motor is a two-stroke, it doesn’t really have valves. I don’t know if it’s possible for a reed-plate to ‘stick’ but they do bend/break occasionally. That wouldn’t make the noise you’re describing, though, just affect the running.

Best of luck.