This is where mislabeling of "adaptors" as "baluns" might be what bit you in the ass in the past.

A balun should have infinite resistance between its analog audio input and its CAT output. This is accomplished with a transformer. That gives you ground isolation, the lack of which is probably why other products have given you hum.

An adaptor incorrectly called a balun -- and they exist -- directly connects a hot lead to one wire of a CAT pair and connects that hot lead's ground to the other wire of that pair. No ground isolation results. Hum is likely.

Believe it or not, a car audio device intended to reduce hum might work for you. It's two audio isolation transformers with RCA cables coming out of each end. I think these wholesale in the twenty dollar range. Jensen transformers will do the same thing, more cleanly, with better frequency response and richer pricing.

Unlike "baluns," you won't need one of these at each end. You'll want to see if you get less hum with it at the sending end or at the receiving end.