Gonk probably can give you a better technical explanation for this, but I suspect you are getting some RF interference on the cables from the converter box. This often happens in 2 channel with an outboard phono stage. The cables are picking up a nearby, high power audio transmission, which might not even be from an over-the-air TV station.

In all likelihood the cables from your converter box to the HT system are not shielded. Get a ferrite ring and wrap the cable through it once or twice, or a ferrite thing that clamps around the cable and see if that stops the RF signal.