Mine was originally in a cupboard and was noisy, then I moved it (see my page) but was still a bit noisy, then I found it was sucking air from the tank selector valve, once I had fixed that it is just a slight rumble.
So hook a hose up to your draw bar then stick it into a tank fill inlet and turn tap on, lay under van and look at the line going to your pump, check for aeration, change tanks if you see bubbles in the line, it may only be slight, then follow the line back till you find no aeration, more than likely the switch valve if you do have bubbles.
Before you do look for bubbles, check the plumbing around the pump if any of it is touching the floor, wall ??? or where it goes thru the floor is a plastic grommet then that could be the source of your noise as the pipes will be drumming against the van body, insulate where the pipes touch and remove the plastic grommet and seal with silastic or softer material.
Ideally the lines to and from the pump should be flexible hose for a foot so the vibration isn't transmitted.
So there you go, hope that helps.
