405 stbd Velvet drive 20-01 noisey

[405driver]

My starboard drive train is making noise somewhere, It's hard to tell where it's coming from. The noise seems to be in the transmission. It only does it while in gear, neutral is quiet. I think I will start with the engine and move back through the plate and then the drive itself. Any suggestions would be greatly appreciated.

[Viper]

Those are all good things to check. What engines do you have? You also want to ensure RPM isn't too low. Also possible flex plate.

[405driver]

Our boat has Mercruiser 454 Mag MPI Horizons. I tried pulling a few plug wires one at a time out of the distributor but I couldn't hear any difference. Will an old dwell/tach meter work on this motor? Thought I might see a drop in RPM with each wire pulled.
I have only had this boat for about a month and am busy solving little issues as I find them. I was looking at the engines a few days ago and noticed that almost all of the bolts in the couplers were loose in some degree. The surveyor seemed to have missed quite a lot of items. I haven't tried since I tightened the coupler bolts, but the port bolts were just as loose as the stbd and it makes no noise. I'll try it in the next few days and post something.

Can one see the flex plate through the inspection hole?

I'm a retired airline pilot, both sides are supposed to sound the same...

Thanks