Have a 2006 Donzi with twin 525 EFIs with about 250 hours on them.

Everything has been maintained properly, etc. and is running great.

This past weekend, when accelerating from a stop, I've noticed the port side engine RMPs seem to lag behind the starboard by about 300-500 RPMs, followed by a bang noise that sounds as if someone threw a wrench at the hull, and then everything is fine. Pressure, temps, RPMs, etc. all in good range and boat operates fine.

Any idea what this might be? Thanks in advance for any insights.

The lagging RPM's is evidence of a cylinder being down. Couple of possibilities but my money is on stuck valve and you tagged a piston. Had the exact same thing happen. Heard a bang, engine ran fine for a while... until it didn't. I would recommend pulling valve covers, rotate engine with remote starter button and watch all of the valves for normal travel. Another possibility is loose rocker or collapsed lifter. Pulling the valve covers will also allow you to diagnose that. First, check all rockers to make sure there is no play between rocker, valve stem and pushrod. Second, rotate the engine a few times with remote starter to make sure lifters are pumped up with oil, then while turning engine with remote starter, press firmly on the pushrod side of each rocker, you should not be able to push the pushrod down. If you can, the lifter is probably collapsed.

Could also be a lean injector which will cause the motor to lay down and possibly a lean pop on acceleration.

A lean pop will sound like a backfire through the carb ( throttle body )
If he is hearing the pistons hitting the valves I wouldn't think it would run long at all.
I screwed up My VP measurement years ago and had 6 out of 8 valves tapping the pistons and never heard a thing and the engine ran great for about two hours. ( dropped valve )

Thanks for the replies guys. I've probably put around 5-6 hours on the engines since I first heard this, it's somewhat intermittent. I'm not the most handy around the engine room so will have a mechanic take a peek at this!

Our last boat had 2005 525 EFIs and starboard engine made a metal ding sound under heavy load, usually when getting on plane and heavy into throttles. Turned out it was the cam sensor, yes the very expensive one that takes place of a distributor.