What ended up being the culprit?
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I turned the High Z on, and the ground lift off. Why these two things made the screeching stop is way above my pay grade, I’m just happy to have stopped it.
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Is phantom power turned on on the DAI? Look on the side.
hmmmmm, I don’t see a switch with a reference to such a thing…
It’s the 48V button between the Gain 1 and Gain 2 knobs.
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No, that button is not engaged. Should it be?
No. Just making sure it wasn’t.
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what is that phantom power/48v button supposed to accomplish?
48V power for microphones that need it
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Why the hell is it called phantom power? It must have some interesting trivia to be the industry standard name
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Yeah I have no idea
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Question
How did Phanton Power get its name?
Answer
Because there are no additional wires to provide the phantom voltage. The same cable is used to carry the mic signal to the mixer, and to carry the phantom voltage from the mixer to the mic. One cable is used as a “two-way street”. Early condenser mic designs used additional wires to deliver the operational power. Thus the method to deliver phantom voltage is unseen, i.e., a “phantom” method.
Actually, the correct technical term is “simplex power”; phantom power is an early recording studio “nickname” for this type of voltage delivery system, and the nickname stuck.
https://service.shure.com/Service/s/article/phanton-power-why-is-it-called-phantom?language=en_US
“Simplex power” sounds like a shady gym supplement from the 90’s
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I like Phantom power much better than Simplex power
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What’s really weird, and making me scratch my head, is that the solution to my loud buzzing and screeching problem, is the opposite of what the Rumble user guide says:
Under the LINE OUT heading
… Pressing the GND LIFT button IN may eliminate hum or buzz resulting from connection to improperly grounded equipment.
It’s illogical to me because I had the GND LIFT button IN when I was experiencing all that racket. When I released the GND LIFT button, the humming and screeching stopped.
Can anyone explain this?
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According to this short explanation, as I’m no expert in the subject, you essentially just re-established a ground.
So you probably didn’t need to have it lifted to begin with. Electronics can be a finicky thing.
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As long as it stopped, Pam 
As least you know what the problem was now!
Cheers
Joe
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I think @Anderson nailed it… When you had Gnd Lift in you had different ground levels on your amp and DAI. By releasing the GND Lift you re-established a common ground.
Generally you want to make sure that your Amp, DAI, and any other signal processing kit are plugged into the same power strip, so that they share a common ground at mains connection. It is possible that one of your components is plugged into a different mains socket… or something has a 2 pin plug and hence no earth… or is your DAI on battery power and no earth connection?
This is a relevant read Q. What is a Ground (Earth) Loop?.
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My DAI is powered via USB which is connected to the same power source as the amp.
I have just been having a look at the docs for the Rumble and U-44 pin-outs and levels. I haven’t found anything obvious. I suspect there is a floating ground level somewhere, and by connecting the DAI and Amp grounds (ie turning off the ground lift) you have managed to get all the signal processing to the same ground.
I can’t think of a way of chasing this further without hooking up some power testing kit.
So my conclusion is: keep all the grounds connected unless you have a particular noise problem.
Sorry I don’t have anything more conclusive for you!
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