Strange whistling noise from the aft end of the boat...

[Norseman]

2001 28TE with the Yanmar.
Noticed this noise, kind of whistling, a bit hollow.
Pulled both belts and checked alternator and water pump bearings back there, all good.
Reinstalled the belts, good tension, same noice, no change.
Was hauled out lately, checked the cutlass bearing then, good as new.
Had a new prop installed, slightly different pitch and less vibration, supper smooth, but that should not cause any whistling noice.
Here is the conditions: The noise is only there while in forward gear and @ low power, from idle 740 to about 950 RPM, then it goes away.
(Tried to record it with iPhone in video mode, no cigar)
Been sniffing around back there while wifey was helming the boat at low power, can’t pinpoint the source.
Speculation on the water lubricated stuffing box, but no indication of anything wrong, good flow as far as I can tell. Maybe will try to shoot the temperature on it next time out.
(The shaft and stuffing box was installed new about 300-350 hours ago, should be good)
Will also focus on tranny, but no indication of leaks or lost fluids or anything else.
Any ideas, any whistling noises at low power and in gear?

[Jkraft]

Dag mine does it too. Not all the time. It comes and goes and it is the transmission whine. I asked two mechanics about it and both told me is absolutely normal. It concerned me too but everything has been fine for me as well.

[Norseman]

Dag mine does it too. Not all the time. It comes and goes and it is the transmission whine. I asked two mechanics about it and both told me is absolutely normal. It concerned me too but everything has been fine for me as well.

Well, ok but on my boat it started after 350 hours of me running it.
(800 hours on the tub when I got it October 2017)

If that is normal, then it should be there all the time.

[RobS]

Did it only start after she just splashed with the new prop? Could be the new prop is singing.

[Norseman]

Did it only start after she just splashed with the new prop? Could be the new prop is singing.

No, pretty sure it was there a few weeks before the new prop was bolted on.
Did not pay attention before as it sounded like an alternator bearing.
Now that I pulled the belts and confirmed the bearings are good, and the noise is there only when in gear, it must be something else.
Engine mounts perhaps..?

[Norseman]

Will answer my own question: Had an experienced mechanic come over and check the engine mounts, they were rock solid.
Also had him listen to the prop “singing” or whatever it is, he could not pin point it, but said not to worry about it..
Ok I won’t, but deep in my mind there is this voice telling me to worry about it.

[Tree]

Cutlass rubbing a bit? Mine did for a while when I fitted a new bearing a while back. Doesn’t do it now but I will need to replace mine again this year after the earlier rope incident as it had a tiny bit of play in it.

[Norseman]

Cutlass rubbing a bit? Mine did for a while when I fitted a new bearing a while back. Doesn’t do it now but I will need to replace mine again this year after the earlier rope incident as it had a tiny bit of play in it.

Nah, the bearing is 4-5 years old, but has 0 play in it, tight as a drum.

[RobS]

Have you ever measured the cooling water flow to the shaft seal? Thinking you have the Tides lip seal...

[Norseman]

Have you ever measured the cooling water flow to the shaft seal? Thinking you have the Tides lip seal...

Yup, got the Tides “Strong” seal.
It was replaced about 500 hours ago, seems to work good and I shot the temperature on it, in case it had reduced water flow and was running hot, but found no indications of a problem.
Never tested the water supply, or flow.

[Luvinlife]

An electric fuel pump that’s on its way out can make a whining sound. Could that be it?

[Norseman]

An electric fuel pump that’s on its way out can make a whining sound. Could that be it?

Nah, very unlikely. The noise is there only at a narrow RPM range and only when I forward gear.
It MAY be there at higher RPMs, but if so, the engine noise masks or blocks the singing. (Sound of Prop?)

[GSSeattle]

This is likely not it, but throwing it out there. I get a whistle on mine that sounds like it's coming from the aft end, only hear it intermittently when going slow or stopped. Took me a few trips to find the source - turns out it was the vented loop / siphon break for the bilge pump discharge, whistling while it takes in air. Could it be at your low RPM / the angle of the boat / any water in the bilge moves and activates a pump?

[Norseman]

This is likely not it, but throwing it out there. I get a whistle on mine that sounds like it's coming from the aft end, only hear it intermittently when going slow or stopped. Took me a few trips to find the source - turns out it was the vented loop / siphon break for the bilge pump discharge, whistling while it takes in air. Could it be at your low RPM / the angle of the boat / any water in the bilge moves and activates a pump?

Good idea, but not in this case: The aft bilges are dry and dusty, not only that but the strange singing noise are always under the same conditions: Low Rpm, forward gear, (nothing in astern propulsion) hot or cold engine/transmission, etc. Always the same..
Lived with it for 6-7 months now, no harm done, but curious what the hell it is.
Keep the ideas coming, thx.

[GSSeattle]

This is likely not it, but throwing it out there. I get a whistle on mine that sounds like it's coming from the aft end, only hear it intermittently when going slow or stopped. Took me a few trips to find the source - turns out it was the vented loop / siphon break for the bilge pump discharge, whistling while it takes in air. Could it be at your low RPM / the angle of the boat / any water in the bilge moves and activates a pump?

Good idea, but not in this case: The aft bilges are dry and dusty, not only that but the strange singing noise are always under the same conditions: Low Rpm, forward gear, (nothing in astern propulsion) hot or cold engine/transmission, etc. Always the same..
Lived with it for 6-7 months now, no harm done, but curious what the hell it is.
Keep the ideas coming, thx.

Well, I tried The whistling from mine was actually the forward bilge pump, but the discharge and vented loop for it are roughly right under the captain's chair. At low speed, and residual water would rush forward and activate that pump. Anti-siphon / vented loop only whistles right when it's pumping the last bit of water / mostly air. It did sound more like a squeaking belt, so bilge pump was not something I thought to check initially. All of this part of the reason I recently installed the diaphram pump on a timer to clear the last bit of water out and keep a dry bilge. (referenced in a different thread)