Curtis, I've used my HT and the main VHF (with masthead 1/4 wave antenna installed maybe a yard from the ST-40 windset) without hearing any problems on either one. I'm only rashly guessing if the ST40 and ST60 wind heads both are the same design, but I think your best bet is a phone
call toll-free to Raytheon/Raymarine, whichever they are calling themselves this year. They're a tech company, so if you explain that you need tech support for an RFI problem, they may have some specific ideas about this.
I don't know what else you've got connected into the mix (maybe a seattalk/nmea converter too?) but if the main VHF, with the masthead antenna, does NOT pick up the interference, and the handheld VHF does, the problem may be in the handheld. I'd suggest getting an antenna adapter to connect the HT to the masthead antenna (typically an SMA-to-PL259 connector, $5-10 from a ham radio supplier). That will give your HT better range and power in case you need it in an emergency
, so it is not wasted.
Now, if you find that using the main antenna "solves" the interference...then you can presume you are getting RFI from something down below, rather than from the wind set.
You can also snoop around for the noise source by taking the antenna off your HT, turning down the squelch, and running it along the instruments and cabling, trying to pick up the signal with your "weaker" HT, missing an antenna. That may localize it.
But it also is possible that something aboard is generating "intermod", a signal that is mixing internally with something in your HT and not coming in via the antenna. In that case, you've got to find the culprit and find a way to filter it--but replacing the HT with a different model may be much simpler, if more expensive.
Borrowing another HT, and seeing if the interference affects "any" HT or just yours, would also be a good way to see if intermod is the problem.