I have tested the 6105 myself in my home tank and I have seen a few returned for noise. Noise problems are not easy to solve, in general, I need to get the pump because the causes of noise vary and sometimes the noise is normal and it is only an issue with the users hypersensitivity to noise. I will outline below the causes I have seen. Please keep in mind, this is a 3400gph pump, dead silence with that volume of flow is virtually impossible. A reasonable amount of noise is a low drone, audible about 4-6ft away, it is a noise you could only hear in a room that is otherwise silent.
1) Magnet holder missing rubber bumpons
2) Magnet holder has a defect, a blister or raised portion which contacts the glass
3) Glass is too thin, magnetic force causes bumpons to compress and magnet makes direct contact, if the glass is less than .5", add a neoprene or rubber sheet or a felt pad from a magnet cleaner to the inside magnet. Out magnet cleaner pad 0220.521, tends to work well for this.
4) Rubber bushing or disk is missing from the drive unit.
5) Propeller housing is not on securely.
6) Propeller blades are damaged or out of balance.
7) Clamp is very tight and transmits vibration, gently stretching it open may help.
8) Pump just needs time for parts to seat, when new, or after opening for service, the drive unit will vibrate more as the shaft and front cover seat, the parts oscillate and create noise, eventually they seat in the ideal position and noise stops.
9) Earlier 6105 with 42mm prop and 24V jumper on power supply, these pumps made a resonance noise that was solved by changing to a 45mm prop and 18V jumper, spins slower but the bigger prop keeps the flow rate the same.
10) Pump placement, corners tend to result in more noise, especially if pump makes direct contact with both surfaces.
11) When changing to the 45mm drive unit the disk or bushings were lost.
