David,

Knowing that the noise is present when the engine and blower are running is good info. In my experience, most calls I get or boats that come in with a hum/hise type noise when the engine is running and have a dual battery bank, almost always leads back to where the head-unit/source unit, obtain their B+. Its usually battery A and the amps are drawing from battery B. Other cases, theres an EQ or similar that may have its B+ run to the same battery as the amps, but its GND is grounded to the helm BUSS. Even though the boats entire ground plane goes back the battery bank and engine block, we sometimes find the need to relocate that EQ ground over directly to the battery and an amp's GND lug. Next up would be the gains, whether its and EQ or an amp, if they are too high, normal noise in the system can be amplified to a point of being audible.

I would start there, then we can did deeper if needed.