I need some help. I have a customers boat with a pair of 496's and one of them under a hard load or trying to get on plain quickly it sound like a machine gun going off in the intake, back offthe throttle a little or when bringing it on plain going slowly it's fine to around 4k then it starts popping. Fuel pressure is 40, no codes, I ran the engine on a separate tank, did a injector drop test on all eight injectors, compression is good, no abnormal sounds coming out of the engine like lose rockers. Any ideas are welcome

sounds like detonation?

The last time that I heard that on my boat, I was missing a few pushrods. Didn't make that noise until under load. Good luck, I hope it's something minor for you. I'd pull a valve cover and take a look.

plug wires and or coil packs...

Stuck valves or timing will do that also.

Typical when a flat tappet cam has severly worn lobes. But, that's not this engine of course.

If all paramters look fine (especially ignition) I would highly suggest removing valve covers (as mentioned) and look for broken valvesprings. if not, maybe springs have lost tension.

Bad valve/valve seat also.

-Do compression test . Oops. I see you did that. By any chance, did any of the cylinders have a much higher reading than the others ?
-Check ignition . Cam/crank sensors. Coil packs. Wires. Etc,etc. TPS / Battery + charging system / all grounds
-Pull Valve Covers .
-Test fuel.

I'll check the springs but remember this is a 496 with about 114 hours and I've worked on a lot of 496's and have never see spring isues even on motors with high hours and ran the motor on another fuel tank.

I once received a brand spankin new GM 5.7L truck engine from local Chevrolet Dealership that had no spark plug holes drilled or tapped on one cylinder head.

Boy did that one give me a buzz kill. LOL.

Point is: never assume anything.

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Was any work done on this motor recently ie: right before this issue came about ? Any other problems customer noticed before this came about ?

All it takes is a little rust on the valve stem after sitting for a while. The pushrod becomes bent or broken when you crank the engine. Usually it's an exhaust valve, but some engines do not discriminate and take out intake pushrods as well.

We see this down here with our high humidity and boats that stay in the water without regular use.

I once nearly went crazy trying to install a new AC spark plug that had no threads! I didn't notice it when I gapped it. (I was just a kid back in those days).