The filter / pump hums during operation
It’s to investigate the cause of this. With pumps and filters positioned inside, you do not need to do anything other than hold the unit freely in the water, keeping it out of contact from any other objects. It is important that the unit remains completely under water and is only touching your hand. If the running noise is now quiet, there is no sign of wear on the rotor, it is just resonance transmission to surrounding elements such as the cover, the base cabinet or anything adjacent to the aquarium. You can then decouple this by making small adjustments.
For external filters, make sure that the filter has 4 rubber feet at the bottom (except for the model CPe1901/2 with castors) and that it is only touching the cabinet there. To test the filter, place it next to or in front of the aquarium on a soft surface (e.g. a thick towel) and make sure that the hoses are not in contact with the cabinet. Many aquarium cabinets act like the sound box of a guitar with its sound hole. The smallest noise is amplified by the hollow body. Therefore, a soft underlay and, if necessary, lateral insulation are advantageous if the resonance is to be amplified. Tubes lying on top or in a very rigid installation with further attachments lead to the same effect. The test with the filter outside the cabinet, as described above, provides information about the actual operating noise.
Strong running noises are caused by a worn impeller, a too strong load on the impeller due to a power reduction (dirt or throttling) and by a too small height difference between the water surface and the filter head, as described in the manual. Another possibility is that CO2 sucked in from the fertiliser system or air from the diaphragm pump, which are introduced near the filter, cause loud running noises.