Yes, it could be exhaust flappers, universals, gimbal bearing, broken tooth in upper. If the seller agrees to it, at least have a survey done on the mechanical side of the boat and save yourself a head-ache and money in the long run. If something is wrong with the boat, mechanical agree the seller pays for survey. If everything checks good and you buy the boat. Money well spent!!!
Mark

What are the odds that either the couplers, u-joints, or gimble bearings are bad in both motors?

Exhaust was my 1st thought, followed by drive gear noise. When on trailer, lots of weird noises.

This is a good point

Random clattering noise is probably(could be) internal anti back wash valves .If you can open the throttle a little and it completely goes away ,makes it more likely .A lot of the aftermarket stuff has no rubber edges on butterfly like merc does ..They can make nasty noise idling ..MO

Drives were down. About 2in off ground. Survey was done last year and everything checked out. I can post results.

If it's in the drive, you should be able to feel the tapping by putting your hand on top of the drive. Did it do it in gear and in neutral?

Easy enough to rule out internal flappers, just stick your hand in there and hold them open. You will get wet, but you will know. Don't stand there long enough to consume CO.

I took 2 little vise grips and put on my flappers so diddnt get all wet I thought there was something wrong when I heard mine .

It was in neutral.

If it has silent choice, then it's probably the anti-splash flappers are located down where the bullhorn connects to the transom assembly. They typically rattle at idle randomly and quit when revved. If it does not have the cast 2->1 bullhorn then other problems.The exhaust tip flappers are possible among the other things mentioned above.

It was surveyed, and the report showed nothing so who knows?