If it's the hardware itself, then it's probably trying to die. Replace it before it does.
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If it's in the recording, it could be anything. Posting one would help a LOT!
The most common cause is a ground loop. Still a hardware problem, but nothing is failing. You just need to fix some bad wiring. More specifically, *too much* connection.
A ground loop happens when you can trace the ground connections all the way around a continuous loop, which usually includes both the signal grounds and the safety grounds of different AC power supplies. That loop is an antenna, that picks up everything, and it just so happens that the AC power line is audible. So that's literally what your local power sounds like.
The solution is to break the loop at some point, while keeping the reference for all of your signals. But,
DO NOT EVER REMOVE A SAFETY GROUND!!! It's there for a very important reason: if something were to go wrong inside of a metal case, and make the case live, then the safety ground provides a high-current path to trip the breaker or blow the fuse. Then the case is no longer live, and what would have been a death trap is no longer functional at all, which is your signal to replace it.
If you've removed that path, then the case stays live, and can electrocute you or someone else. The ground of a signal cord is not enough to provide that high current; it may become a fuse itself, and keep the danger, in addition to frying the cord.
That signal ground *is*, however, strong enough to electrocute you. There's a story about a big-name band almost losing their lead singer because the mic was AC-live. He kissed it like you're supposed to for the best signal, and he collapsed on stage.
So,
DO NOT EVER REMOVE A SAFETY GROUND!!!
The correct solution is to break the *signal* ground, NOT THE POWER SUPPLY'S GROUND, while still getting the signal itself across. That's usually done with a transformer for analog low frequencies like audio, but there are other ways for high frequencies or anything digital like USB, ethernet, MIDI, etc.
- The standard for ethernet, by the way, does have transformers built into the jack itself. High-frequency transformers can be that small.
- MIDI is opto-isolated, with an LED that shines on a light-sensitive transistor, all packaged up in a light-tight plastic case.
- USB is not isolated at all, unless you intentionally put an isolator on it, but it's interesting to see how that chip works. It's almost a complete radio set on the same chip, minus the antennas.
If you buy something to break the signal ground, it can go by lots of different names, depending on the brand and how their marketing team felt about using the actual term for it or not. So you have to read between the lines sometimes, or buy a more expensive one that checks all of the engineering boxes.
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Or, if your power cords are spread across a bunch of outlets, it might be enough to just put everything on the same power strip. You still technically have a loop, but it's a lot smaller, and it's the size (specifically the 2-dimensional area that it encloses) that determines how much noise it picks up. If it drops the noise below audible, you're done!
