1999 Volvo 4.3/SX? outdrive thumping

[Gator5713]

First, Id like to thank everyone for all the info I have received from this site over the last couple years working on various projects!
While I am certainly not a master mechanic, I am a well qualified mechanic (by both trade and hobby) so I am by no means scared of tearing into something that I don't know much about as long as I can get the required guidance to know what I'm looking for...
On to the issue.
(new to me) 1999 Larson lxi with a Volvo/Penta 4.3 pushed by an SX-M outdrive.
Boat has been running perfectly all season, but last weekend when I put it in the water and put it in gear I noticed a 'thumping' that increased/decreased with speed and went away as soon as I took it out of gear. It was just noticeable enough that when my wife got in the boat she asked 'whats hitting the bottom of the boat?'
Everything still ran and operated perfectly except the thumping which neither got any worse nor any better.
My guess is that I'm likely looking at Gimble bearing or U-joints but figured I'd ask here and see what Y'all thought and any advice I might need as I go to inspect the outdrive this morning.
My plan is to start by running the motor on 'muffs' and engaging the drive to see if it will re-create out of the water and let me narrow down its location.
Thanks in advance.

 

[Scott Danforth]

Start by pulling the drive and inspecting

 

[Gator5713]

Where the trim cylinders connect to the outdrive seems to be missing a bushing and has a fair amount of slop/play. Would it be wishful thinking that this could be the issue?
I know, I know... I need to pull it apart and inspect/ service everything anyway just hoping for an inexpensive fix to keep on the water as a more expensive problem may put me out for the season.

 

[Scott Danforth]

Fixing the "more expensive" issue is cheaper than a "catestrophic failure leading to a sunken boat"

 

[Gator5713]

I didn't say I wasn't going to fix what's wrong. I'm trying to find what's wrong and it doesn't cost anything to be hopeful that it's an easy/inexpensive fix.
Gear oil in the outdrive is good. Prop removed so I could put the flush muffs on, fired it up and put it into gear and no thumping/grinding/anything out of the ordinary which brings me back to my question about the trim cylinder connection bar having play.

 

[Gator5713]

Stopped by the local boat shop and store. He confirmed that I am absolutely missing a few bushings and given my description it is possible that is the cause of my thump. He also said it didn't sound like the u-joints or gimble bearing but did say it could be a chipped tooth in the gearbox and to check for metal in the gear oil and inspect the gears.
Drained the oil, extremely minimal shavings on the plug, oil black and gear oil stinky, no signs of water, metal, etc.
pulled the shift cover and inspected the gears and they look nearly perfect!
So, unless someone has had a similar experience that can point me towards something I'm missing it looks like I'm waiting for my $20 worth of bushings to come in and heading to the lake!

 

[Gator5713]

Go ahead with the "I told you so"...
although I'm still convinced the missing bushings are the cause of the thump; bummed that I'm down waiting on parts I convinced myself that pulling the drive is too easy NOT to do at least for peace of mind. Water in the bellows... I knew they were old and needed to be replaced but stubbornness said it'd wait. Amazingly, everything is still in tact and no play! Greased the ujoints and gimbal but figure probably ought to go ahead and replace the gimbal while I'm in there and have to order a gasket and orings anyway. The water inlet oring was apparently pinched and leaking since the last time it was off and the gasket was permatex...
here's to convincing the wife I need a few hundred in parts!