Need help diagnosing strange noise - '99 21v

[mdk681]

Any thoughts on what this noise could be caused by? It changes pitch with RPM and disappears around 1,400rpm. It sounds just like the ringing you get when you run your finger around the rim of a wine glass. You can hear 2 different tones in the video. It only happens when in gear moving forward. Only happens at lower speeds. Any suggestions? It has been driving me NUTS! I am planning to buy a mechanics stethoscope to try and pin point the problem area but am looking for suggestions where to look. Any suggestions help!

Video link:

https://youtu.be/g7v7crOcEyg




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[ChrisSnow]

Any damage to the prop?

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[lee]

What engine do you have? I know some of the scorpion engines made a whistling sound around that rpm.

[mdk681]

Prop was damaged a few seasons ago, pretty much the entire driveline was rebuilt. Prop was repaired and did not make the noise the first season after rebuild.

Engine is 350 Mag MPI with around 600 hours on it.

I have never heard anything like this before, I'm totally stumped and can't find much online anywhere. It's such a strange ringing/high pitched frequency. It almost seems like it's a pulley bearing but I feel like that would be more of a constant squeal.


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[JCP]

Try running it for 20-30 seconds without the belt to see if the noise is gone. Easy way to verify if it is a belt driven noise.

[mdk681]

It only seems to do it when the boat is warmed up to temperate. At first I thought it might be the prop whining, I don't have any vibrations though.

[IDBoating]

Are you certain it only happens in forward gear? Never with the transmission engaged, never in reverse?

If it does also happen in either of those circumstances, I'd check the spark arrestor. Ours "sings" sometimes as the incoming air resonates with the little fins.

If it's truly only in forward gear: I'd check the propshaft thruhull. Not sure what kind you have on a boat of that vintage, but if it's a "seal" style (not a packed gland style) and it ran dry, the faces could be scored and could resonate like that.

[mdk681]

I have cleaned the spark arrestor a few times over the years, and I know that noise well, it is not that.

I only hear it in forward gear and it is VERY noticeable. I am now wondering about the through hull. A couple seasons ago when I smacked a rock, almost everything was replaced. New strut, prop shaft, dripless shaft seal, rebuilt prop, rudder, steering cable, etc. Is there any way to inspect the seal to see if it is indeed trashed? Is there anything that can be done to rebuild this seal or does it need to be replaced entirely? Here is a picture of the shaft seal installed a couple seasons ago:
IMG_0461.jpg

[Bryan]

Maybe a shot in the dark, but could the drive plate be slipping a little bit? I'm thinking that's what my boat mech called it. He said the springs start to weaken 600-700 hrs in. I didn't hear a noise from mine but I had the tranny rebuilt last year and he went ahead and changed the plate too because he said it didn't look like it'd be long...

[Ruger761]

Can you get in the locker while someone else drives see if you can pinpoint the location a little better?

[UNSTUCK]

I'd try running it in gear on the trailer. Then you can climb all around it and have a good listen. A good home made stethascope is a broom stick or similar. One end on the part you want to listen to and then make a fist around the other end which is then pressed against your ear. That way you can touch all over the engine, trans, drive with out being too close to it. Works great.

[IDBoating]

[new] dripless shaft seal.... Is there anything that can be done to rebuild this seal or does it need to be replaced entirely? Here is a picture of the shaft seal installed a couple seasons ago

Good, you have a modern "wet" shaft seal. The fact that's it's the wet style reduces the likelihood of what I suggested, but it's still worth checking.

Yes, you can get the parts and do not have to replace the whole thing. But the even better news is that you can often reverse the stainless ring to use its "fresh" face, essentially doubling its useful life. When mine got damaged that's what the dealer mechanic did, and it's worked great ever since. Obviously this won't help if the carbon ring is scored, but it's worth a look.

As for confirming this: I was probing around in the engine compartment with a mechanic's stethoscope and happened to touch the rubber bellows on the shaft seal - and the sound vanished! The compliance of the bellows allowed the ring(s) to oscillate just enough to make noise. Touching the bellows changed that relationship enough to silence it. I would try that... while the boat is in gear and making the sound, gently probe the bellows and see if the sound changes. Costs nothing to try, and might reveal the answer.

[ChrisSnow]

I'd try running it in gear on the trailer. Then you can climb all around it and have a good listen. A good home made stethascope is a broom stick or similar. One end on the part you want to listen to and then make a fist around the other end which is then pressed against your ear. That way you can touch all over the engine, trans, drive with out being too close to it. Works great.

You will burn up your cutlass bearing if you do this for long. Needs water as a lubricant.

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[UNSTUCK]

Does he not get the need lubricant from his dripless shaft seal?

[IDBoating]

No, at least not reliably. The water sourced to the shaft seal lubes the shaft seal - it keeps water between the faces of the carbon and stainless rings. Some of that water perhaps runs down the propshaft and gets to the strut bearing, but not nearly as much as when the boat is floating in water - which is what the strut bearing expects.

You might get away with it a bit better by having a garden hose flowing freely onto the forward end of the strut bearing, so that water can flow through the little channels machined into its ID. But in general it's a bad idea to turn the propshaft under power when on the trailer. I've done it for a VERY short time, but I'm talking less than 30 seconds and only at idle RPM.

Here's a photo of my shaft seal water flow when on the trailer. There are tiny little drips running down the propshaft, but you can see the vast majority of the water is just pouring down onto the trailer crossbeam. That strut bearing is definitely not getting enough water in this situation.