Please help me eliminate the hum my digital rig creates


I've noticed a hum coming from my system lately.  It's there whenever amp is powered on, regardless of which input is selected, and hum increases as the volume is turned up.  Today, I isolated the problem to my digital setup-- whenever I disconnect the digital rig from the amp, or remove the SPDIF (Audience Au24-e) between the Bluesound Vault 2 and the Mytek Brooklyn, the hum disappears. 

system: https://systems.audiogon.com/users/dkidknow

I have upgraded PCs powering both the Vault and the Mytek.  My power conditioning is a cheap belkin single rack unit, but my turntable and phono preamp are both plugged into the same belkin, and there's no noise created there.  I'd appreciate your suggestions on how I can get rid of the noise.  It's driving me a bit batty!
dkidknow
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We had the modem replaced today and every other component remained the same plugged into the same place and the hum is completely gone. It was just a faulty modem that began to generate hum when it went bad. In response to Jea, I live in Manhattan in a large apartment building, so all grounding issues are out of my hands and only licensed professionals are allowed to work on anything in the building.  NYC’s department of buildings views property owners as a huge source of income through penalties and fines so I’d be surprised if everything wasn’t strictly to code.

Thanks everyone for their thoughts and time spent responding to me. If you hadn’t asked the questions you did, I wouldn’t have been inspired to get more scientific and really eliminate each possible cause of the noise to discover its source.
Congrats! Glad you found the culprit. I had a modem do that to me too, although it somehow started pouring audible rfi into my tube preamp. Bizarre when it happens suddenly.