Hum in Speakers with New Dedicated 20 amp Circuit

@jea48 I believe you.  I was probably confounding the local code requirements with the NEC.  My mistake.

@jea48 from decades ago, I thought I had learned that metal conduit alone wasn’t good enough due to likelihood of the conduit being imperfectly connected or becoming disconnected vs. ground wires.

Now I am wondering if this was something local to Massachusetts?

One equipment grounding conductor. (Note: If a steel conduit is used between the main panel and sub panel that meets NEC for use as an equipment grounding conductor.)

That's a surprise! We can't do this for junction boxes or outlets.

Bring your panel up to code before worrying about ground loops.

Also, while we are at it, looks like you have mixed brands of breakers in there.  The breakers have to be either of the same manufacturer as your panel or specifically rated to be in that panel. 

Make sure you use cover plates for any holes in the cover.

Welp, I thought this requirement was a lot older, but per this discussion, 1999 was when the requirement changed:

Looking at the picture, the ground and neutral from upstream were wired to the bus on the right.

If an electrician did this, get a new one.

OP:

The bare wire from the main panel is the ground.  It seems to be connected to the buss on the left.  The white wire from the panel appears to be connected to the bus on the right. 

Keep that separation.

This is a sub panel.  Ground and neutral should not touch here.

Also, check the usual suspects.  Did you also introduce or mess with a coax cable to a set top box or modem?

The ground and neutral should only come into contact at 1 place in your home, at the service entrance.

If this is anywhere else then this is bad.  Check the N to G voltages on your circuit with everything disconnected.  Should be zero.