Ferrite ring on power supply cable of Tomy baby monitor receiver unit.
We were given this system and from the off it would hiss and pop and crackle and even temporarily cut out. Martin and Vicki have the cordless version, and when Martin mentioned that theirs does the same when the receiver is in the charging cradle and it wakes him during the night, the penny dropped. It’s mains borne interference and signifies to me a cost-saving measure by Tomy resulting in an inferior product…
I had a ferrite ring somewhere, and sure as anything I found it when I was looking for something else. I followed ‘general’ advice of wrapping the cable around the ring five times and hey presto, a fully functioning receiver with no horrid noises. Hooraa!!
I found someone on ebay selling ferrites and ordered a couple, including one for Martin & Vicki and they can now sleep without white noise, pops crackles and bleeping.