150 VMax Noise

[Valmet412S]

My 2001 150 VMax Carb has a knocking type noise which appears to be coming from the midsection of the engine, particularly the port side. It is not very loud and only audible in neutral or at low rpm idle. The cadence of the knock seems to be slower than a typical engine knock but does speed up as I accelerate until it either stops or is covered by engine noise. I checked the compression and all 6 cylinders are around 105 when the engine is cold. All spark plugs were a little dirty with carbon but were wet with oil. I have experienced no loss of power. I am thinking my next step will be to take the lower unit off and run the water hose into the water pump and see if the noise persists. Suggestions?