Moon Eye is a 2004 IP370 #21. Her Yanmar 4JH3E with 1480 hrs has developed a high pitch whistling sound at 2300 to 2700 rpms. She is quiet and smooth running above and below that range. It only happens in forward, not neutral. We are southbound on the ICW from Maine to Florida. Adjusted stuffing box. That did not solve it. Motor mounts were replaced and engine aligned after that a year ago. There was no whistle before that and none until we started putting her to hard use on this trip. Cutlass bearing was ok at recommissioning in June. Comments?
Do you have a vented loop coming off your mixing elbow? If so, there is a small vent at the top of the "U" pipe. That vent can "whistle" when the water flow is fast enough (ie: higher rpms) if its not working properly or cracked.
We are original owners of 370-24 also with the 4JH3. First, Google "singing prop" and read some other owners experience and the science behind the problem. Then my story. It may or may not of course apply to your boat.
For the first 4-5 years of ownership every Spring after our haulout and with a completely clean prop we had a high pitched "singing" between 1300-2200 rpm. Within a few weeks it would go away and only return in late summer after the diver had cleaned the prop and wiped the bottom. Several mechanics took short rides and heard the sound - checked alignment, motor mounts, but no change.
I then by chance the next Spring mentioned this to the owner of the marina where I haulout and he said I think you have a "singing prop". We pulled the prop and I took it to the Norfolk prop shop and the owner there said I can cure this by a slight camber on the trailing edge and actually gave me a copy of one of the scientific papers you will find online. Indeed he did - problem gone.
The other comment about the raw water vented loop is also interesting. It reminded me that Island Packet issued a factory service bulletin for our 370s to fix a problem in that the hose from the raw water vented loop to the bilge as installed at the factory did not continuously run downhill, allowing water to block air flow. They sent me a new hose and a few other parts and the new way to route the hose that I installed myself. If you are not the original owner you might check to see if that was done on your boat.
Enjoy your ICW trip. Done it twice and some wonderful places to stop and enjoy along the way.