Can't speak for Martin's experience (obviously), but in my two loop setup, there's not much of a perceivable difference between 1 vs 2 pumps in operation (at the highest speed setting). They are, however, fairly loud, with some high pitch whine noticeable, which to me is moderately annoying - this perception, of course, is highly subjective, I have no measuring equipment to quantify this in any way. The case (ATCS 840) is about 1.5m from my listening position.