Thanks for replying! Just to clarify, you see this some days with your scope, but not every day? I've only had this scope a few weeks so that would validate my observation that it suddenly started. Are your DMM's bench type plugged into the mains, or on battery?
It's definitely coming through the probes. If I disconnect the probes from the scope, the noise drops to almost nothing. But I wonder where it's coming from? I tried turning off everything in my office and it didn't make any difference.
I'm wondering if it's related to the ground. The mains ground is directly connected to the probe ground, so it runs the length of the probe wire. When I unplugged my UPS from the wall so the scope is running on the UPS battery, the noise changed significantly (see the attachments). At this point the scope was basically ungrounded, but it's also getting a stepped sine approximation for power instead of a true sine wave, so I'm not sure if that's the cause of the noise change. Maybe I need to dig out my true sine wave inverter and see what happens when it's completely isolated from the mains ground. Hmm...