sellout007: Depends. just sitting here I can think of several different scenarios, each with a different solution.
1. Humming noise caused by the pump vibrating on mounting surface... Solution would be to put the pump on foam or a mouse pad. Two of my pumps sit on a doubled up piece of underlayment I had left over from doing my pergo floors. And the last pump sits on a 1/2" piece of hard foam.
2. The drain and/or return line are hitting something causing a humming noise. Adjust PVC pipe.
3. The PVC is too short or too long causing the any number of issues and the pump hums. Replace with a piece of spa-flex.
4. No known reason but the motor hums through the pipes. Again use a couple feet of spa-flex.
5. And in my case, do to the fact that the water falls down about 5' into the crawlspace and hits two 45s real quick, I could hear water noises. I insulated the pipes with foam and it cut the noise down by 75%.
I'm sure there are more possibilities and solutions, I just can't think of any right now. I will tell you that I have a hum from the return pump that is noticeable in the downstairs bathroom and upstairs in the kids bathroom and our bedroom. It's not too bad, traffic outside is louder, but I am going to replace about 5' of my hard line PVC with spa-flex to cure the problem, which is that the return pump sits on the wood as it enters the crawlspace from the wetroom. I just can't seem to get it up and off the wood without causing another vibration.