My octo nwb150 skimmer too is making rattling noise in the sump. I decided to open the pump connection so removed it and put it in a bucket of water. Disconnected pump,
I didnt know how to open it fully so I can check the impeller, but opened the inlet /outlet knobs...saw nothing wrong no debris. Flushed them , ran pump alone. Put it back up , ran skimmer in bucket for 1hr, there was no noise. Then I put it in the sump and here is what happened:
There was no noise initially. After a few mins it slowly started as a minor 'rrrr" and then to a loud intermittent "RRRR" . It increases as the water fills
up the column! After some time I lifted the skimmer by 2", the noise reduced a tiny bit. This time I made sure that I can see the pump clearly near the glass. when the sound is bad and intermittent, the pump doesnt choke nor there is any visible interuption in the bubble/jet flow from pump into skimmer.
Trouble is , even by moving ,shifting I cant figure whats causing the vibration. Wonder if its an issue with resonance? As Mike suggested I can try mousepads but clueless where to put them.
My 15gT is sitting on top of 2 (stacked)wooden pieces. Each 3 ftlong,1.5" thick and 12"wide. Could it be that the base is not very stable?
How do I id /fix the problem?
Pls share

Thanks-kris