Philco Predicta predicament

[ShartZenith]

I have a Predicta that a friend helped me get working last Winter. The picture couldn’t be much better except it slants down to the right ever so slightly. The nagging problem is the audio.
Sometimes there’s a constant humming sound that is loud enough that it overshadows the TV audio. It’s not on any particular channel and very intermittent. I have noticed it tends to start doing the humming if the show goes to a commercial or changes to another feed of any kind. Even when I intentionally put up the signal strength screen from the converter box it starts to hum, then goes away when I take the signal strength screen off.
I can also tune out the humming with the knob behind the channel selector, but that doesn’t always work and sometimes when it does quiet down the humming the picture is no longer ideal. We’ve recapped everything and replaced numerous weak or bad tubes when we worked on it last winter. What am I up against here? I will share your answers with my friend who did the lion’s share of the work on the TV and is the “techie” type. I’m not. Thanks for any and all help.