Hagen

In general noise can be the result of vibration from the impeller rotation. In some cases this can be caused by a piece of debris becoming lodged in the impeller well and causing the impeller not to rotate smoothly. Have you disassembled the unit and removed the impeller form the impeller well to clean it and remove the built up debris that can fall into the well during normal operation? Remove the motor by twisting it 1/4 turn and removing it from the Filter Case. Then remove the impeller from the impeller well and ensure there is no debris or dirt trapped in it. The best way is to run it under a strong flow of clean tap water and if possible use a small round brush to ensure all dirt build-up is removed from the Impeller well walls as well as the Impeller itself. Never use soap or detergent when working with anything associated with the aquarium as trace residues can cause harm to the fish.

While doing the cleaning, run your finger into the impeller well to ensure the sides are smooth, not warped or bumpy. There should be no slime or protein wastes attached to the impeller or the impeller well after they have been cleaned. The impeller should be able to rotate freely in a circular motion when power is applied, and the impeller well is filled with water. Running it dry will always be quite noisy, and the impeller will bounce quite a bit, so this is not a way to test it at this disassembled point. An impeller will also not turn freely by rotating it manually by a finger when no power is applied due to the magnetic forces of the impeller - these will allow the impeller to turn, but only in about 1/4 - 1/2 rotation.The only real way yo see if the problem is solved is to assemble and test in operating conditions.

Inspect the impeller for damage the center hole should be round and even, not oblong or appear worn away. If the impeller has become worn, replace it - the impeller is not covered under warranty. Impellers do wear over time and may need replacement to rectify a weakened magnetic assembly or damaged blade(s)

Once the parts of the impeller and impeller well are rinsed clean and debris free, reassemble the filter and test, in most cases the noise that was a result of debris build-up is gone. If the problem persists, refer to the manual, or you may need to send the unit in for repair. You can find access tot he various equipment manuals and your particular AquaClear Filter at the below link:
http://ca-en.hagen.com/usefultools/instructionmanuals