Engine Thump / Knock

Thursday morning we were cruising out to Block Island from CT, all was good until about 3 hours in I heard a noise from my port engine. Immediately idled down and shut the engine off to check it out.

No change in performance, RPM, oil pressure, temps, anything just the noise. It’s a thumping type sound coming from the intake (spark arrestor). I took a stethoscope to it and from what I can tell it does not sound like it’s something hitting the valve cover but I can hear it if I put the stethoscope on the spark arrestor area.

The boat will be back at my home dock tomorrow so I can dig deeper into it, but I wanted to see if I can get some where to looks first. First plan is remove valve covers and inspect.

300HP 350Mag MPI horizon motor
https://youtube.com/shorts/Yl4RHnd_XWE?feature=share

Sounds like compressed air escaping from one of the cylinders. That could be from an exhaust or intake valve, damaged spark plug, or leaking head gasket.

Lots of noise there so a bit tough to tell. If an exhaust valve fails to open due to a bent pushrod or failed rocker, it can make a popping sound out of the flame arrester when that cylinder's intake valve opens. May want to pull the valve covers for the price of a couple gaskets to look for a problem there.

Trying to discern from the engine rpm. If it's valvetrain and/or rod knock (not so likely), it would be even with the engine cycles. Check the timing chain cover directly behind the engine water pump. I had a nylon-toothed cam gear stick a tooth in the cam chain. While it was cyclic, it didn't coincide directly with the engine's cycles, if that makes sense. Since the chain's length doesn't evenly divide into the engine pulses, the noise came at a much slower pace than the engine cycles.

Have the spark plug wires been disturbed recently, like a tune up been done?

Agree with JimmyBuoy. That noise is absolutely cyclic and even with the engine speed.

I would disable / ground out / disconnect the spark plug wires, one at a time, and see if noise goes away. At least that will tell which slug is making the noise, then go from there.

Also hear some metallic noise in the background, kind of faint, but sounds like valve train noise.

Any update on this? Curious.