Long cables can act like an antenna and pick up RF interference. This can occur with cables that are either shielded with a cheap foil shield or not shielded at all.

The hum could be from mains power if you are running your cable close to an outlet or parallel to where the mains cable runs through the wall, among other things.

A guitarist in a band I played in a while back had a guitar lead that would pick up radio stations. His nickname quickly became Radio Moscow because of it.

He replaced the offending lead with a high quality shielded version and it solved the problem.

are they powered monitors? if so, are the both plugged into the same outlet? different outlets can have different paths to ground. this makes for ground loops.

Thanks Kurt - I do have them plugged into the same power strip. However, you made me think of something - I think I will swap out the monitor's power cord with another power cord just in case the ground is flaky with that power cord - although the power cord works fine in the other room - but it's worth a try.

Thanks Sean - but (from above) I swapped cables to each monitor and the same monitor still hummed. The cables are high quality/shielded/balanced.

Bring the humming monitor back in and connect it in the old position where it doesn't hum. Then use a 3 circuit power extension cable to connect it to the distant outlet instead of the close one. If the hum comes back, then you have a simple ground loop. The local and distant ground potentials are different. The shared ground via the signal cable creates the loop, and the small voltage difference induces the hum. If this is the case, simply try breaking the audio ground first, disconnecting the screen, leaving the ground unconnected. Note NOT lifting the power ground, that's there to keep you alive. Find an old cable you can sacrifice, or re-terminate after the experiment. Sometimes different outlets are fed from different phases, or from distant ground points. If you find the grounding is the problem, then the other solution is to run the audio and a power feed from the silent socket.

Thanks Paul and Kurt! Yes - It was a ground loop. I ran a power cable from the silent socket as you mentioned and that solved it.

Great forum of people all around on here.

If you feel happy using a voltmeter, one thing worthwhile doing for a safety perspective, now you cured the problem audio wise, is to measure in the AC range the potential between the two grounds. A few volts in normal, but if you get anything above ten volts or so, something electrical is askew, and it would be prudent to get somebody in who can diagnose what is going wrong, because the point of grounding is to create an equi-potential area. The idea is for the ground to be the same potential everywhere on a site. We have tough rules in the uk, and this issue concerns us when people run long cables that go outside this area, outside especially. Lots of people use protection devices for safety, but grounding is a primary safety feature. If your grounding is dodgy, it's worth fixing.

That all said, the design of some buildings means that a difference in potential will exist. Especially factories where they have three phase motors. Most studios, however, are not factories! I've had guitarists complain their lips tingle when they touch the mic. Tube amps, thrown in and out of trucks can often have their ground sitting at half the mains voltage. No current, but it still is unpleasant. In the UK this means 100 plus volts, and a small spark.

We get good at breaking ground loops, and having lift switches on DI boxes and other kit, but they shouldn't really need to be there, should they?