ODD vibration? grinding?

My wife and I just purchased a 2000 Disco II 75,000 miles, we love the rig but we have one problem. It makes this noise that is a little more than a hum but you can feel it if you press your feet down firmly on the floor. The sound is hard to locate as it vibrates thru everything. Its there when its in park and when driving. The sound changes with the RPMs but doesnt follow them exactly. I dont know if this makes sense to anyone?
:banghead:

I had this checked out by a mechanic who works mainly on rovers, he listened to it and said its normal. It comes from the ACE pump, its a vain style pump and as soon as it gets a little wear on it it makes noise. Nothing to worry about tho!