I don't know anything about the spec of your van but I can tell you that a diaphragm water pump makes a noise something like you describe. So is it a pressure switch or a pump making the noise?
I have spent some time trying to reduce the noise made by the said pump on my van. Check the mounting points of the pump. I used some thick rubber to isolate the pump from the board it is mounted on. Sticking pads to the board it is mounted on should cut down drumming. Is there space to put a box over the pump, something heavy, lined with absorbent material would help,
Not aware of any alternatives.