Printers, crt displays back when they were a thing, cheap chargers... even low-end LCD displays. All of them emit a high pitched frequency that can be maddening.

Interestingly it's pitched above the range of my tinnitus so I get to hear both most of the time :/

Summer here is my favorite time of year for that reason - the insect noise gets so loud at night, and falls right into the range of the tinnitus and e-noise such that it completely obscures them. It's almost like those sounds aren't there for a few months every year.