Here's a few steps you can take to eliminate it or at least minimize it.
* Try lowering the amplifier input gain.
* Try an RCA ground loop isolator between the head unit and amplifier.
* Remove the source unit positive and negative from the factory harness and hook those up to the amplifier pos. and neg. at the amplifier and not at the battery. If you want to maintain the keyed function then add a relay.
* Ground the pump and pump relay to the engine block with a larger cable and quality lugs.
* Try a polarized 16 volt 10,000 MFD capacitor across the pos. & neg. of the pump supply or pump relay.
* Make certain that the batteries have good ground connections to the engine blocks.

Sorry I don't have better news. It's alot of effort. One or multiples of the above procedures usually work. Hope this helps.

There are resistors that you put on your trim pump...no ground issue, you can run grounds all day long,nothing is going to change. The Part number is #8M0034258 this is a Mercury Marine part you will need 2 and here is a picture, again you don't have to redo any of your wiring unless its just not done properly...
Keep us posted:drink:

try installing a stiffening cap on the power side of your amps, they are likely pulling too much power from the overall system!!! assuming of coarse your grounds are all ok...