The AC coils in the relays will hum and it is the nature of the beast. You may try blowing out the contact area with compressed air. They will tend to vibrate more if the contacts do not mate well due to dirt.
I believe the best solution is to change the transfer switch to one that uses relays with DC coils. This is an example. I'm not endorsing this particular unit just letting you know that they make one with DC coil relays in order to avoid the hum.
http://rvpowerpartsplus.net/index.php?main_page=product_info&cPath=11&products_id=131I did the following to reduce the hum of the relay AC coils in my unit and it reduced the noise by 30 to 40% in the coach which makes them still audible but usually not annoying. I cannot say where your unit would end up noise wise with this treatment given no two sets of these relays seem to produce the same noise intensity, nor does it stay constant.
I removed the relays by drilling out the rivets and mounted a furniture glide rubber pad under them and bolted them back down to the box. I did not torque them very tightly given that I was trying to isolate the noise. I also placed a rubber pad between the metal box and the "sounding board" the unit was mounted on in the coach. The project took about 4 hours and given that it reduced the noise but did not completely eliminate the issue I would probably opt to change the unit out if I was to do it again given what I know now.
You never know the outcome of an experiment until it is done.....hope this helps.
Later Ed