Researchhound: To answer your questions I think it depends on the cause of the issue. I also have a loud pump, but it seems to quiet down considerably after I run the water for 1-2 minutes. I did try the things you listed trying to solve my vibrating pump.
I purchased one of these silencing kits and attached it to the pump.
Than I took an old mousepad and put it under the pump.
Then I got some cheap insulation from the local hardware store and covered my pipes where they were touching each other, the floor, wall, etc.
It helped quiet it some, but I think my cause must be getting air into my water system.
I haven't quite figured it out yet, but since I did the above 3 things, I still get some loud vibration. It seems like after I fill the fresh water tank, the system is loud. After I run the water from the sinks for 1-2 minutes, it quiets down to what I would say is "normal" (based on my last trailer pump).
All 3 things you mention are cheap and worth a try.