I agree with Griff, post your solution !
Several members have taken the time to respond, so you in turn should post what your solution was !
This web site is worthless unless you do your part , then many others will do a search on the same problem and fix their problem with minimal effort.
Boating in the high performance area is extremely costly, and unless you are a millionaire, most people on here do their own work , that is why this web site is so great !

Depending on transom angle this is not necessarily true. The alignment is checked to ensure the relation of the engine to the transom assembly and gimbal bearing is correct.

I had what may be the similar noise on my 32 regal with Bravo 3's. Warming it up at 1500 RPM's, with the drives turned, I could hear a rattling/knocking noise. Took it to my dealer, they replaced u-joints, gimble bearing, the noise was still there after they essentially threw parts at the problem (both of those items were less than 2 years old). Took it to another dealer, they knew the issue almost immediately. They removed the drive, verified alignment was on, greased the input shaft and replaced the two rubber o-rings on the input shaft. The service manager advised me the noise would probably be gone for 2 years or so. He was exactly right. The boat was silent for at least two years before it crept back in.

With the majority of boats it is true. The U joints have to be inline.