Re: VHF noise / squelch
hmmm Splitters....... hate them (my own thoughts) should have 1 input 2 outputs (some have more outputs) the thing is very simple. the more connectors you have, the more chance something can and will go wrong.
For what they cost an SWR meter is a must, if you havent got one, they can be bought for around 30UKP (40 something USD) and are worth 10 times this amount with what they can tell you. For example not only will it tell you if the antenna is good or bad but if the radio is putting out the correct amount of power as well. Now if you get a bad SWR more than 2 to 1, well its on the output side of the SWR meter, but it could be damaged coax cable, or a bad antenna. I have often found that it is usually the coax that has gone bad, sometimes it is the antenna but more often than not the coax has got water logged. It is amazing how fast that stuff can drink water, you wont believe how much of the stuff it will hold and still look like new even though its now damaged beyond use.
So my thoughts are, remove the splitter try the old antenna and coax and see if there both OK (if you have/borrow a SWR meter check with that as well), possibly find theres an issue with a plug or the coax is water logged. If these check out OK, then its either the splitter or one of the connections to the splitter that's at fault. If its the splitter take it back and swap it for a 2nd VHF antenna and have one antenna for the VHF and one for the AIS transponder, much safer way of doing things.
Hope this was some help, my best guess the coax or one of the connections is no good.