Note that now AquaClear and Fluval are owned and run by the same company and even use the identical motors just with different impellers. Based on the part number C2-C4 & AC20-70. (AC110 uses a different motor.)
Note: I have two C3's, a C4 and an AC50. i have never once cleaned the impellers or motors on any of them .... or torn any of the filters down. I have crystal clear water. But change the filter media religiously. As much as I hate my AquaClear ..... it's for an entirely different reason:
AquaClear Bypass Issues
My AC50 (Jan 2014) was dead silent right out the box ... not a peep .... never a problem with noise. My first Fluval C3 (Sept 2013) was so annoying and rattling I could not stand it. Everyone on the forums said "ignore it for a few weeks." They were right. It became dead silent with me doing NOTHING. My second C3 was dead silent right out of the box.
My C4 has always been quiet but not silent. All of a sudden after two years .... just this week, it sounds like there are air bubbles in it that make a rattling sound .... but it goes off and on.
Suggestion: since you bought your AC
used ......... there could be many excuses for loud noise. If the prior owner used sand as a substrate, consider your filter TOAST ... and move on. I'd recommend a Fluval C series ...... but it is always luck of the draw.
I was at a neighbor's house the other day and noticed one of her aquariums (55 gallon) that had both goldfish and turtles in it was filthy green looking ....... with a noisy AquaClear70 ........ while the other was a 20 Gallon High was SILENT
with very clear water ..... running an Aqueon filter with tropical fish in it (mostly tetras). While the water was not "pristine" by our standards, to most people it would look crystal clear.