I have a 1998 Wellcraft 28' 2800 Martinique with a factory installed Marine Air Systems Vector Series self contained combined heat/ac unit with a digital passport II control board.  The boat is kept in Salt water on the coast of New Jersey near Long Beach Island.  This is my second year as the boat's owner and am having a problem with a noisy compressor. 

The unit is located directly under the forward cushion of my convertible dinette.  It is located adjacent the forward v-berth.

My problem is a deep humming noise when the compressor is running.  To try and describe it, it sounds like someone with a deep voice repeating the word, waah, waah, waah, waah, waah, over and over constantly.  It only happens when the compressor is on and quiet with just the fan on. 

I don't use the heat so only have noticed it with the A/C on.  It is a very deep and loud and annoying noise. 

I am not familiar with these units and don't know how the compressor works so I don't know if there is a part that could be replaced or not. 

Any advice and/or help would be appreciated.  I have thought about trying to sound proof the locker the unit is located in but the sound is so deep, it almost seems to be resonating through the hull. 

I can upload a video to youtube this weekend of the noise if need be. 

Thank you for your reply and sorry for not responding sooner.  I have answered your questions in bold below.  

Did it make this noise last season ?

Yes.  The unit was installed over carpet and over the winter I removed the carpet and installed it directly to the wood.  I think the sound may be a little louder this year as the carpet is not in the locker anymore to muffle the sound.

What kind of water flow do you currently have overboard ?…Low water flow will cause it to work much harder. 

I have a good strong flow of water.  When I first turned on the unit this season, I noticed it did not start pumping water.  I turned it off and checked it over and over.  I tried sucking on the line and blowing water back in through the drain to "prime" the system.  I eventually just tapped on the pump and it started working.  I did not run it for more than a minute or two without water flow and now the pump kicks on every time. 

Has anyone added refrigerant lately ?…Overcharge can cause laboring too.

Refrigerant has never been added since new.

Have you been out in any rough seas that pounding may have caused a change in its mounting or mounting position in some way ? 

Mounting position is strong and secure and mounted in identical spot to last season.

Is the board/shelf, or deck it's sitting on securely fastened down, and is the unit itself screwed securely to that ? 

Yes, the board it is fastened to is the floor of the cabin and the unit is very secure.  There is no buffer between the unit and the floor.  What do you recommend to install under the unit?  Dynamat? Rubber?  Foam? etc?  What thickness, density, etc?