So have you totally isolated the fact that it's the pump, and nothing resonating that it's attached to (including tubing)? If so I can't offer any suggestions, since a noisy pump is a noisy pump, unless the impeller is not set properly or manufacturer defective (warped, etc). You may consider taking the pump apart and see if you can quiet it down with a rebuild.
I'm assuming this is a new pump. If not an obvious suggestion is same. Take it apart and see if impeller or pump inside is calcified, thus needing a good overnight vinegar soak to de-calcify it.
If it doesn't hum when running alone (ie not attached to anything), thus caused by your tubing or sump you may have to play around with placement to avoid a sweet humming spot like I had. Things attached acted like a resonance sound box), and I eventually just gave up on glass sump, and went with a Rubbermaid. Silenced everything.
But before I converted to a soft Rubbermaid. I did manage to stumble on a silencing technique. My sump was the problem. Once day I was doing maintenance on the tank and placed something heavy (A small piece of plywood with a 2 Gal Bucket of water) on top of one corner of my Glass sump. To my surprise hum drop like 70%. That kept me happy till I went with a Rubbermaid with sound down 90%