Have you taken the impeller apart to make sure nothing is wrapped around the shaft? Really check the top of the motor where the impeller sits, even the tiniest debris (like sand) can be irritating.
If you have thoroughly checked everything else, I would try replacing the impeller. Sometimes if a Whisper is buzzing, it can mean the impeller needs replacing. Replacement parts can be found online or at most chain pet shops.
Also, it is most likely a Whisper 30 or a Whisper 60 - A 30 will have one filter cartridge and a 60 has 2. Unless you have a newer type (No flow adjustment on top of the intake tube), those have different denominations (sorry, not as familiar with those, but I'm sure you can find the model on a pet shop website). A newer type also uses a different impeller than the older type.