Topic: "Sizzling" sound during playback

A few days ago I started hearing this annoying noise during playback.  It happens the same whether I playback from Cubase, or Windows Media Player or Youtube.  There is no noise when playback is paused, it only happens when an audio signal is actually playing.

The sound is kind of a buzzing or sizzling or hissing, a little like the sound a loose snare on a drum kit makes when it's caused to vibrate by loud music playing in the room.  It's not very loud, but it's noticeable, and seems to increase with higher frequency audio.

I'm using a RayDat connected to a Behringer ADA8200. Analog output from the ADA8200 goes to a Mackie Big Knob, and from there is routed to headphones and a pair of Event TR6 monitors.

I suspect the noise is coming from either the RayDat or the ADA8200.  Any suggestions on how I can troubleshoot which?