Loud whine or hum?
 

Loud whine or hum coming from the back of the boat comes up through the bottom of the boat, only happens once the boat is on plain and seems to be the loudest at about 2500 to 3500 rpm appears to go away or its not as loud as rpm increases. Trimming up or down has no effect, trim tabs seem to quiet it down a bit if I lower them. I was told the upper drive has only about 150 hours on it, it was replaced with a factory mercruiser rebuilt upper. Just changed the drive oil it looked brand new and no metal shavings replaced it with ams oil marine gear lube. Checked gimbal bearing and lubed drive shaft checked alignment when it was off looked and felt fine. The boat is a year 2000 baja boss 232, 454 mag mpi, bravo drive, labbed 23 pich mirage prop. Any ideas would be of great help

usually i would say gimbal. But maybe its just the prop. its hard to say because everyboat ive been in usually has some sort of hum. Check all fluids and lube, try a different prop

Is it an XR or Bravo 1?
XR straight cut gears will whine, but usually the helical cut gears are nice and quiet.

If you hang over the transom and listen, does it seem extremely loud??
With the drive off, does it turn freely with some drag?

It is a bravo one, yes it does spin freely with some drag. When i take the boat out next i will try to listen off the back of the boat. the gimbal felt very smooth and free when i turned it by hand. I think i can get my hands on a different prop to check. Other than the noise the boat runs great. Thanks for the responses