Sounds like you might be ready for a new mechanic, or at the least a second opinion, no offense. Sounds like you are close to Clustergear, might reach out and have a conversation with him. Got to know him last fall. I replaced a pair of tired XR's with new and sold Clustergear the take offs. Guy is the real deal, has some serious history with Merc Racing, lots of hands on experience with drives. If I needed a drive fix I would take the drive to Lake George to see him. Just a thought on my part, he's a decent guy.

Many have said swap the drives and while they are off like I said stick an alignment tool in -- check the alignment and then run the engine to see if the alignment TOOL makes a perfect circle. If not like I said - bad alignment - or loose coupler or its coming apart. What do you have to lose at this point ? And that takes like 5 mins all to do since your pulling the drives anyways to swap over. At the very least I would check both couplers - looking maybe for some other issue that can be just starting as well.

I will do the alignment tool test before i swap the drives. I do remember removing the drive awhile back and starting the engine and there was no noise....I will keep you updated..I have to bring the boat back home to do all of these tests..especially because of the drive jack..what a savor lol...This noise has been there since last summer after messing with the u-joints.. I remember first when the clip popped out of one cap on the u-joint
as i just made it back to the marine. Putting the drive high up getting it on the trailer I heard an awful loud metal to metal noise. I immediately shut it down. Took the boat home and called the guy that put the u=joints in. It took us 1 hour to get the drive off because the u-joint clip popped out and the u-joint cap popped up jamming into the castle nut on the drive. Just a little refresh of what went down when all this started...But looking at the castle nut.. No Damage Was Found

I will keep in touch Thanks

I pulled the noisy drive off and did the alignment bar test.. Looked fine to me (see video). Then i mounted the noisy drive on the other motor (startboard) and the noise was there. So the noise is in the drive. see attached video. I then pulled the top cover off and rocked the top gear back and fourth...This is the noise i am hearing while running.. see attached video.

Any suggestions???

Can you send me his name and #..
Thanks

no flappers...straight out transom

Great idea but i used some large towels instead..

I sent 6 videos to Mercury Marine..They said they couldn't hear it from a video..But they said check the top cap bearing and if you haven't ever changed it, you should just be careful on the tower height. Also make sure the + - are lined up perfectly on the gears. top minus bottom plus or visa versa, and something about the shift linkage adjustment. They also mentioned something about when the engine comes around on the compression stroked it actually slows down and speeds up even if we don't hear it and even though the engine is spinning smoothly it can cause a rattle, an engine that is tuned differently or needs to be adjusted. I told him I have 490 hp and i have the only boat out of 12 performance boats in my marina with this noise and the other 12 boats have a lot more hp than i do.They mentioned boats with transmissions that have a clutch plate rattles. I believe i mentioned something about this in a prior post. I swapped the drives and the noise followed the drive. My other drive is quiet, so i will cut out the rotation of the engine diagnoses.
Took the boat out yesterday...no vibrations..Hit some big rollers, came out of the water a lot.. Put the boat on the trailer, started it up and the noise was a bit louder I guess from the oil thinning out.

Well, thanks for all the input from everyone..It was a good try..Hopefully the drive stays together. Does anyone need a complete input shaft assembly..Cheap 10 hrs on it.. here's the link Thanks Jay
https://www.ebay.com/itm/263814894879

Got it thanks
J