My buddy's Formula 312,, stbd eng. 502mag MPI.. every now and then makes a ZZiiiiiiinnnngggggg sound. then gone. No loss of oil pres. The belts are good. Even cut oil filter to check for any metal. Eng. runs fine..
WTH can this be?? It's happening more often now he says.. but he can't catch it.. Come OSO'rs.. I told him someone here will know..

Maybe a belt pulley or idler?

Good thing it isn't a ZZZAAAAAAAAAAANNNNNNGGGG sound, that would be bad........ Catch it on a vid maybe? I have no clue, maybe a bearing in the tensioner going?

Now thats funny,, I hate ZAANGS,, Yu0 would think if it was a tensioner it would be constant.. But he did add a kick a-s stereo,, and since its been doing this more.. maybe the new alternator load is causing this..from time to time...hhmmmmm......nnaaaaaaaa can't be???
F-it maybe he just needs new 525's.

Starter slinging out?

Had an old lawn mower that did this and it was a zing and not a zang sound.

This zing and zang talk makes me thirsty for a bloody mary!

run boat without cell phone

+1... More than likely the bearing.

Check the starter!

Possibly a broken return spring in the starter solenoid, allows the gear to contact the flywheel intermittently.