First off, tap it. Sometimes my whisper 20 makes a rattling sound but I tap the cover and it goes away. If that didn't work, turn off the filter, take out the tube, and reset it inside the impeller thing tightly. If the previous did not work, take out the tube and inspect the impeller to see if anything is lodged anywhere. Again, if the previousdid not work, take apart the whole filter and wash out the inside of the filter casing with a hose to get rid of any sand or grit in the impeller hole thingy. And of course put the media in a bucket of tank water.
If the last one did not work, call Tetra.