Random radio sources, but a small part of the signal is CMB. I wasn’t sure what you even meant by thermal noise but I believe you’re talking about flatscreens. I found something that said it was “similar to snow on analog TVs” - so I gather there’s a difference.

Funnily, Google AI says, “In the 1940s, people could detect the CMB at home by tuning their TVs to channel 03 and measuring the remaining static after removing other sources. This allowed them to prove the Big Bang before scientists did.” So they had that going for 'em, which is nice.