If you.search around you'll find everything from tin foil to ground lifts to cabling to power filtering ...

Do free stuff first.

Unplug the cables from both monitors near the monitor side.  Then move the monitors to the opposite side and plug them back in.  Did the noise follow the HS8 or does it stay on the left side?  If the noise follows the HS8 then you may need to focus on the HS8 itself.

I say move the monitors rather than moving the cables because the audio cables may be crossing an aggressor and rerouting may change this.  This is a bigger problem if the cables are unbalanced but none of this really matters if you follow the suggestion.  And power cables should not follow the HS8 either.  You want the left to be plugged swapped to the current right's power and audio and position.  And vice versa.

After gathering this data point you may try plugging each monitor's power into a different outlet/strip/etc to try to impact the grounding topology.  That's assuming the noise didn't follow the HS8

If you have a ground lift plug use it on one or both monitors (the buzzing one or buzzing and clean one).  If you don't have these then they're inexpensive.  If you have a lot of spare (extra) power cables I guess you could clip the ground leads off of a pair of these to use as ground lift cables.  I'd advocate more for the non destructive approach.

Swapping cables, all that jazz.

Buying stuff (cables or conditioners, or gadgets)

Warranty exchange ...

Maybe shielding (if it's just the one HS8).  Likely a labor cost item for someone who knows where to put it.  Otherwise shielding can be inexpensive if you know where to put it at risk of voiding warranty.

Really, you're tasked with getting a more detailed understanding of what's going on and chase the right path to fix the issue.  There is likely paid talent available locally (to you) that can help.  

1. Power off EVERYTHING except the left monitor. Don't start disconnecting or moving anything until you first check the monitor itself to see if it is the problem.

2. Do you still hear the interference? If not then power the other stuff back on ONE thing at a time until you either hear the interference or it is all powered back on.

3. If you still heard the interference then disconnect the monitor and take it to a CLEAN power source (another building if necessary) and plug it in to see if the monitor itself is the problem.

If the monitor has a transformer/other internal issue it will usually show itself just by having ONLY the monitor on with no other power sources available.

A monitor should not be making that noise on its own so it needs to be fixed or replaced. Simply adding shielding isn't going to address the underlying cause. The 'solution' isn't to reduce the noise volume so you don't hear it - it is to eliminate the cause of the problem. 

As mentioned by others interference is often due to a bad/wrong cable or cable routing issues. So turn off all other sources that might be using the same power channel. If you still hear the interference then you may have a dirty power source or cable 'somewhere'.

I'd offer the same caveat here - don't assume there is just ONE simple cause.

1. Did you add any power source? Such as moving a plug/source to a different outlet?

2. Did you add any new power drain? This can be as simple as turning on a piece of equipment that hasn't been on recently or adding a new piece of equipment.

3. Did you add/move any cables recently? Moving, or stepping on, a cable can cause interference where it didn't exist before.

4. Do you have cables that run in parallel? That makes it easier for a problem in one cable to infect another cable

5. Are there exposed cables in the traffic area? Just stepping on a cable can break it or cause problems.

If the monitor is bad either replace it or get it fixed. Just having audible noise is likely the LEAST of your problems. You need to know if the 'noise' is infecting the actual signal or being recorded.

You are the only one that knows how important the quality of your actual signal is and whether you can afford to just 'mask' the problem or if you need to find, and eliminate, it.