[Jayce]

You could either have plates rubbing on the tuner or they could have grown 'whisker's that are touching between plates. I had a Crosley with the latter issue as they used rather cheap steel in their tuning capacitors. Never really did figure out a repair. Like you said with your RCA, once you landed a station it played great.
The first thing would be to check for dirt or dust between the plates as that can cause noise. Also, (how do I explain this?) I cured one radio by spraying contact cleaner where there the moving part of the tuning capacitor makes contact with the copper wipers or contacts. Someone else can probably explain that better than me!