Try these tests
While in the slip nothing running--turn the
wheel as hard as you can.
No noise? should elimenate the rudder and all of it's hardware.
Gain access to the engine compartment and wiggle, shake, push,twist and pull every thing you can on the engine,starter,air cleaner,fuel
pump maybe something rubbing on the engind drive or trans output. Check linkages, if you can see it try to move it.
Nothing, eh!
I was worth a try.
Use you eyes and nose. Things that are runing out of lube or aru rubbing usually will turn blue or smell like they have gotten hot.
Now make sure you take off you watch and rings and neck tie and long sleeves.
Start the engine.
While ideling and cold put the trans in f-n-r- any noise. Some times something will show up while cold.
None?
With the engine ideling and warming up touch the NON rotating stuff and feel for grinding and also look and listen.
Put the trans in gear and feel and listen again, one more time in reverse.
If it is inside the boat you should be able to find it.
No luck? Try a little bit of power
Still no luck?
Try taking the boat in calm water. I could be you picked up something on the prop early while running in choppy water and will now show up in calm water.
Time for a swim