🚀 go-pugleaf

This is a ground-lift issue - it's been reported several times before, and there's a long thread about it in the old archive. The solution is simple - you need to keep the cablebox ground away from your local ground connection, and one way to do this is with an isolation transformer on the RCA outputs. This isn't an issue with your TV connected - this isolation is already provided within the set. But the cable box (like your DVD player as well, incidentally) isn't grounded until the incoming cable is connected, and the ground for this will be some way from your house, and what you are getting is all of the noise between the two ground points added to your signal (and probably a hum as well). So, to fix this properly you either need isolating audio transformers for the RCS connections (as above), or you could ask the cable co. if they have an isolating transformer for the incoming cable - which they probably have, just for the people with this particular problem.