Stereo hiss/ background noise.

[macdaddy]

Ok crew.  I have had this boat for several years and it has always has background noise when the motor is running.  It is constant.  Does not change with motor rpm, but only makes this noise when motor is running.  I have a switch in the dash for when I want to use the stereo, so it is not wired to ignition.  I know it is grounded to the main grounding post under the dash.  How do I get rid of this noise?  Need help, but do not understand stereo talk.  Do I need to ground head unit directly to the battery...positive and negative. Help me out but please use simple terms!!  Thanks.

[MLA]

Quote

I have a switch in the dash for when I want to use the stereo, so it is not wired to ignition.

Its tied to the ignition more than you may think. The ignition key constant battery positive, B+ for short, is likely coming off the same feed as the dash switch gets its B+ from. Just think of the ignition switch as, well, just another form of a switch then the rocker. 

Dual batteries?

External amps? 

[macdaddy]

I have dual batteries and 3 amps.