It does sound like a mobile phone signal to me (can only just hear it with headphones in your sample). I’ve never noticed it in a recording, but I do turn my phone off just in case, I suppose you’re not immune to others close by though. When I play my jukebox in the car through a cassette adaptor, it picks up the mobile signal quite strongly.
I don’t anything about the cables involved to say whether it’s something that can be eliminated or reduced, but good luck with it.
My phone (amplified), attached.