[bassman]

What does the pump sit on? Concrete? Hardwood? Carpet?

Coming from the live sound industry I have learnt that one way to isolate vibration (noise) is to have as little material touching the floor as possible.
For example on large speaker systems sometimes the cabinets are mounted on spikes. Usually 3 spikes instead of 4, like a tripod. Or they are suspended from cables. Sometimes they are hung in slings as well. That way the cabinet is touching absolutely nothing.

For my pump I built a plywood base, slightly larger then the mounting plate on the pump. Then I drilled a hole in each corner of the plywood and inserted 6" long button head machine bolts with lock nuts to adjust the height. The pump is then bolted to the plywood. Now the entire thing sits on 4 points. I would have done 3 but I wanted the extra support. Eventually I will be using a rubber mouse pad and cutting 4 little disc to place under each leg (bolt). My pump stand sits on concrete, that is why I am going to make the rubber discs. If yours is going to sit on carpet you may not need them.

Now spikes would have been a better choice than the bolts but I couldn't find any long enough.

I would say it cut the noise from my pump down by 80% atleast.

Another thing is to make sure your pump and all plumbing attached to it is not touching anything but you probably already knew that though.

Hope that makes sense, if not let me know and I will take a pic of mine.

PS If this seems over kill then just grab a mouse pad and try that, get the thick, dense rubber type.