Hi,
Jon- "If it were me I'd put the old return pump back in the sump and hook the new pump in a closed loop. You're not going to hear anything (waterwise) in the closed loop and you don't have to worry about a new overflow."
I agree 100%.
"If the pump itself is noisey, sell it and get an Iwaki."
I'm not sure how the noise of Mag36 would compare to its Iwaki counterpart. (It would take a Iwaki 100($353) AND a Iwaki 70($287) to do 3600gph). But generally speaking, Iwakis are good pumps.
If you're looking for high flow and want it quiet,. You might consider the Dolphin Ampmaster 3000($287). I know they had some problem with the seals awhile back, but this may have been fixed.
:D