OK... just wanted to pass this along in case it helps someone searching the archives.
I've got a Mag3 pump attached to my Remora Pro skimmer. Lately, it's been a bit on the noisy side, so I took it off my tank last night and cleaned it. When I hooked it back up, it was noisier than ever. Like, "I'm ready to throw my tank out the window" noisy. I'm sure OSHA would've shut my house down for exceeding industrial noise levels. After taking it apart and putting it back together a couple more times, I got it quieter, but still obnoxiously noisy.
In a better frame of mind, I took it apart again today and thought more about the problem. Here's the solution that made it purr like a kitty again...
The little rubber cylinders on the ends of the impeller shaft need to fit snugly in their little holes in both the intake housing, and the motor housing. In mine, for some reason, the rubber cylinder fit really really loose on the motor housing - enough that it could cause the magnet on the impeller to rotate out of balance. I'm guessing that I had a good amount of calcium buildup in that little hole that kept everything fairly snug until I tried to "fix" things by cleaning it.
Anyway... I disassembled the impeller by pulling off one of the rubber cylinders. I then hotglued the other rubber cylinder (attached to the ceramic shaft) into it's hole in the motor housing. I then reassembled the magnet and impeller on to the shaft, and installed the other rubber cylinder and screwed back on the inlet housing. The hot glue filled up all the extra space and kept that shaft nice and tight within the housing. It's nice and quiet again.
In looking for a fix, I noticed a lot of folks complaining on various forums about "rattling" mag3s, and I'm guessing this might be their problems. If yours is rattling, before you go out and buy a new pump or new impeller, check the fit of those rubber cylinders (bushings) and give the hot glue fix a try.
Oh... and in my search, I found an AWESOME mod that only takes 10 minutes to do that makes the Remora so quiet, you'll wonder if it's working! No kidding... I've done it and I can't tell you how happy I am (as well as my wife!) I'll post that and a link to where I found the quickie mod within the next hour or so.
edit: Here's the post where I describe the Remora mod, in case it gets lost... http://www.aquariumadvice.com/viewtopic.php?t=96396