Is the projector on a different circuit breaker than the A/V equipment? If its on a different circuit breaker are the two circuit breakers on the same phase? ( You want them to be on the same phase.) Generally you can determine this be counting and determining if they are odd or even. Start counting from the top left going down the left side, skipping over the larger 2 pole breakers. Do the same thing on the right side and start your counting over from the number 1. Even numbered breakers will be on one phase and odd numbered breakers will be on the other.
They are on different breakers.
Is a 2 pole breaker when there are two switches in the space of one breaker? Or when there are 2+ breakers that have a bar connecting them?
You don't have to move the projector, but use the extension cord to plug it into a different outlet in the same room as the projector to prove that its not a problem with that particular outlet.
How about if you use the extension cord to plug the projector into another outlet in the same room? Different rooms? Room with an outlet on a different phase?
I plugged the projector into another outlet in the same room and I still have buzz. I'll try plugging it into a different phase if you answer my question above so I can figure out how to count the breakers.
Otherwise, since it's a new outlet, then you or the installing electrician should know where the other end of the cable supplying the outlet originates. Which would give you a head start on refeeding that outlet from the primary A/V circuit.
P.S. A diagram of your A/V system would be helpful.
That electrician is long gone. Wouldn't come back just to answer that question and probably wouldn't remember because it's been a while.
The AV for the theater is quite simple.
Outlet #1: AV Receiver
Outlet #1: external amp connected to AVR w/ RCA cables
Outlet #1: DVD Player connected to AVR w/ HDMI (uses 2 prong AC plug)
Outlet #1: Chromecast connected to AVR w/ HDMI (powered via USB using 2 prong AC plug)
Outlet #1: HDMI transmitter balun connected to AVR w/ HDMI (uses 2 prong AC plug)
Outlet #1: Bass transducer transmitter connected to AVR w/ RCA cables (uses 2 prong AC plug)
Outlet #2: Projector
No AC: HDMI receiver balun connected to HDMI transmitter balun over Cat 6 + connected to projector via HDMI
Outlet #3: Subwoofer connected to AVR w/ RCA plugs over coax (uses 2 prong AC plug)
Outlets #2 #3 are in the same room.
As I understand it, only devices plugged into ground can be ground loop culprits and the only ones are the AVR, external amp and projector. Everything else has an ungrounded plug -- but all of those things are plugged into a grounded power strip on outlet #1.
The speakers that buzz are the only two connected to the external amp. The ten speakers connected to the AVR do not buzz. It's a bit odd considering the AVR and external amp are plugged into adjacent plugs on the same power strip, but speakers to the AVR don't buzz.
All this got me thinking there might be an issue with the speakers plugged into the external amp, so I tried different speakers and I still got a buzz. I also tried a different external amp (also plugged into outlet #1) and still had the buzz. I tried a different set of cables between the AVR and the external amp and still had the buzz. And unfortunately this is my second AVR (first one had a malfunction) and I had the buzz with both.
I think doing all of that was a waste of time since I know plugging the projector into Outlet #1 with an extension cord solves the problem so there is some issue with Outlet #2. But at least I have conclusively eliminated any other factor from being the cause.