hi - am using an aquabee to drive my skimmer and a Eheim for the sump return
to tank.
Experience: Aquabee is very quiet while the Eheim seems to vibrate a lot( and it does generate quite some heat ).
It's the vibration on the pipe and sump tank that cause the noise - eventhough you may seat the pump very tightly and put in cotton/sponge to absorb the vibration.
if you do a search on "hum, humming" etc on any of the other forums you will probably find similar reasons.
cheers