Engine bang

[Waterdog ls]

1999 moomba outback LS.
I just put in a new motor in my boat because of marina doing piss poor work.
Anyway I have been going through the breaking period and have about 7 hours on it.
So I have tried to get up to full speed just recently. As I hit 40 mph I get a sound like metal releasing.
I was thinking it could be a alinement issue with the motor, trans and the shaft.
Dealer said it sounds like a flex plate.
The motor and boat sound and work great until 40 when it makes that sound. I've only gone up to that seed twice and each time it made the same sound. I pull back on the throttle after the sound is made so I don't know if it would continue making the sound. Any thoughts and help would be greatly appreciated thank you.

[Fastest1]

You installed the motor? Do you remember the condition of the flex plate? Specifically the springs? I would have to say I would think a flew plate issue would be noticeable at any speed. The procedure for shaft alignment is a sticky here. Did you follow that process?

[Waterdog ls]

I personally did not install the motor. But the motor runs smoothly and quietly The Flex Plate from what I understand it should be making noises once I put it in gear if the flexplate is bad

[Fastest1]

Just being familiar with the design to some extent. I cant imagine it creating a pop at any one time. Upon engagement or slowing down might create a single thump as the slack was taken up. Have you tried to take it up to the same rpm without a load? Be in the water for that test for engine cooling. Any play in your prop shaft? Bushing intact?

[Waterdog ls]

Took the cover off the boat and the sound was totally different. Turns out it was a backfire