Help - intermittent noise

Odd noise tonite while out on the lake. 1999 Mariner Optimax 150. Out running around at about 4000 RPM and starting getting a knocking noise from outboard. Back off and noise goes away, get back into e throttle and noise comes back. Back off to idle, let boat slow down, punch it and no noise, but then sometimes a few seconds and sometimes a few minutes before noise coming back. Always seems to be when on plane cruising around. Any thoughts? Doesn't seem to matter what RPM I am cruising at the noise eventually shows up. Could it be a clutch issue? Running a high five prop and no previous issues but have not had the boat very long.

Thanks for any help.

I'd start by taking a real good look at the air compressor. Pull the belt off and spin it by hand. See if you have any play anywhere.

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Originally Posted by gregpro50

I'd start by taking a real good look at the air compressor. Pull the belt off and spin it by hand. See if you have any play anywhere.

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DITTO.. how many hours are on the motor.. .they have a tendancy to fail after 300-500 HRS...

Try a different prop. Could be a harmonic imbalance between the prop/driveshaft/crank/flywheel. Have seen some give the symptoms you are describing because of certain SS props. The older inline 4 cyl's were notorius for it. Some of them you couldn't even run a SS prop. I have seen some Opti's do it too.
Worth a look before you dig deeper. Changing a prop is easy just to check it.
Also check the belt tensioner. They had a bad habit of breaking the boss of the block :eek: !

b0atguy

I thought I would try a new prop tomorrow, but I will definitely check the compressor and the tensioner before I go out.

Would any of these issues cause an intermittent noise?

Forgot to add local dealer scanned it when I bought and I was told 22hrs. Boat has been sitting for last 6 - 7 years.

does the motor jerk when the noise is present or only make noise?

It does not jerk, only makes noise.

Check to ensure that your flywheel is tight.

Looks like I wont be able to get out on the water today to try a different prop. I did check the air compressor, no bearing issues, no slop in the pulley. Check the belt tensioner- no problems there. Checked the flywheel, it's tight.
One thing I did notice was that the intake for the air compressor was missing the grommet for the flywheel cover. Do the air compressors ever go into a unload state where air could exhaust out the intake casing the cover to thump up and down? Just a thought. Don't want to do too much arm chair diagnosing with out going and trying a different prop as b0atguy suggested.

had one doing same thing not a opti but wound up being the gearcase lost one tooth off the pinion very hard to diagnose just a random clunk or knock sound