i have had horrible luck with finding a quiet pump in that GPH
i actually just bought and returned an AC50 pump as the 3rd try to find one that doesnt rattle.
my rio800 rattles the least...
an CA800 rattles like holy hell
and the AC50 rattles too
CA600 is quiet as a mouse though... all of the pumps have been used to return water from a sump to a 29gal tank (supplimeting a rio1100 which is quiet)
ive givin up trying to find a quiet pump.