Some pedals that have timers in them like a flanger or chorus will generate noise and feed it back through a power source. Had this happen quite a bit a few months ago and actually had to remove a couple of Boss RC-2 looper pedals. I really liked both of them but they were generating a high frequency noise through my tube amps which I couldn’t hear myself because of my age and also hearing loss, but my wife could hear it and said it was terrible. I disconnected the pedals, put a battery in them and tried them by themselves. No noise but when I tried to use them with other pedals with the battery power still on, they produced the same type of noise, so I just quit trying to use them. Changed power supply but same problem. Some pedals will interact with other pedals and you just can’t use them.
Hope this helps. Sincerely, Fingerstylepicker.