Gimbal bearing or u joint?
 

I recently noticed a growling sound when running. Worse at speeds above 40 mph. Changes, but doesn't go away when wheel is turned, or when trim is adjusted. I have twin engine setup 2004 bravo xr's with Stellings extension boxes. Drives where gone through 2 running hrs ago. Pretty sure it is gimbal bearing or u joint. Is there a simple way to diagnose the problem, or to distinguish port from starboard? Can this be easily diagnosed without running the boat? Boat is in lift so I will need to run it to the launch to get it to the shop. Thanks!

Pull your drain plugs on your drives and look for metal first. You may have a gear going south.

run it on the trailer trimmed down and see if you can hear it if you can pump the gimbal bearing full and see if it goes away