Sounds like a jet engine hum.

[Magnarae]

I ran out to Montauk and Block Island for the Holiday week. As I was pulling into Montauk Inlet, I noticed what sounds like a jet engine hum. Its not coming from the engine. If I am at idle in gear it sound like as whooshing sound coming from under the boat. I also noticed it as I was at idle in gear with following seas. Any ideas?

[Mark Deeser]

First thing that comes to mind is an extrusiom. Sounds are a little water like water intrusion, they can present themselfs quite a distance from their origination point. I wonder if the sound is only heard at idle because of the lower general noise levels. The hum does not paint a mental picture in this case, but a whoosh is possible air or water. I would look for constrictions, clogged filters, ( thinking air ) or even an issue with the turbo. Most interesting problem, hope others with same equipment can provide better insights. Thanks, Mark.

[craw_daddy]

Could be your turbo waste gate is stuck open..

[Magnarae]

how do I check to see if that is the case?

[craw_daddy]

Your turbo boost gauge -if you have one, otherwise a diesel mechanic can confirm it.