Weird. Is it a 60hz hum? Sounds like a grounding issue if it's coming out of the speaker. Plasma displays can make a noise as well, some kind of feedback. Maybe check to make sure everything is properly grounded?

Definitely tighten all the screws holding all the boards on the backbox. I had a subtly hum once in my old whirlwind and that seemed to do the trick (if I remember correctly).
Does your DMD cable have that magnet clasp around it?
I may be wrong but I believe that is there to "absorb" any stray current from the ribbon cable that could interfere with the speakers?
LTG is a good resource to reach out to with situations like yours.
He's helped me numerous times over the years.
I feel like I should adopt an animal and name it LTG in his honor

It's only humming because it doesn't know the words.

Is the DMD a Cherry-branded one, by any chance?

If you turn off the game, completely disconnect the DMD, and then turn the game back on, is the hum gone?

I have TOM, and it definitely does not hum. I think jsa is on to something with the grounding issue.

One way to check the capacitors in the game is to look at the surface of each capacitor (the top, flat side of the cylinder). If there is any bowing out of the capacitor, they are failing and need to be replaced. Given the pinball machine was sitting for six years unused I wouldn't be surprised if this was an issue in multiple places. That being said, I'm originally an audio engineer, and hums were often related to bad grounding (which is why tightening the screws can help, since in theory the pcb panel is well grounded).

I'm sure there is a proper way to test capacitors with a multimeter, I'm just electrically challenged so someone else can comment on that.

Mine hums as well from the speakers. Anxious to see the solution

"Check the grounds" means check all the ground connections. Each board has at least one ground connection. Sometimes a break is caused by a cold solder joint, a bad crimp on a pin, a broken wire, dirty pins/connectors, etc.

Use a multimeter in diode test mode to check for continuity.

Its your transformer. Williams changed the position from the bottom of the cab to mounting it to the back of the cabinet, from memory people put rubber under the transformer mounts

You would have to look at the schematics. They are wires in the connectors.

My TOM did the same thing, I tightened all the screws holding the transformer in the back of the cabinet and it all but eliminated it.

You mean it didn't help, right?

No no, it reduced the humming to almost completely gone.

That is correct ..
Put some insertion rubber and that should solve the issue with transformer Hummmm.
i have just been to lazy to do mine ...

Does the actual sound in game raise or lower on it's own without you touching the volume? I had a Stern SM that had a nasty hum, but the sound was messed up along with it in the way described above. After much messing around with tightening down screws etc...I found it to be that someone installed the wrong type of diode on one of the main boards. After replacing the part with the correct one the hum went away and the sound returned to normal. My point is make sure that no one modified the boards with the wrong component. Sometimes hard to find, but can cause this type of problem.

Is the hum one consistent sound that doesn't change in pitch or volume after the game is on, and independent of whether a game is being played or not?

Do you have LEDs? Could be Electromagnetic Interference (EMI) caused by some. LEDs and poor insulation

It is a natural rubber, with one cloth layer (inserted)
or 2 cloth layer depending on the thickness

ebay.com link: Strip Rubber

Last week i bought my TOM and also i had the same issue as mentioned here. Two kinds of humm. One higher pitched from the speakers and a lower more irritating, which indeed as kiwipinhead suggested, came from the transformer.

When you moved the transformer a little when powered on you could hear the hum changing.

So i googled on transformer hum issues and found a post on a Belgium pinball forum from a guy having the same issues with his JM.

He removed the T shaped metal plate which some (and mine) WPC’s have. The transformer is connected to this grounded plate and connected to the cabinet with four bolts.

So when i removed this plate and reconnected the transformer the hum was completely gone!