[JRTJH]

It sounds like the diaphragm in the regulator is "humming/vibrating". That could be caused by a bit of moisture collected inside a hose while changing the tanks, a defective "safety valve" that won't fully open in one of the tank lines or even a "safety valve" in one of the tanks that isn't functioning properly.

I have a "clicking" in one of my tanks "at times" depending on propane use. If it's just the refrigerator, it clicks, turn on anything else and there's enough flow to overcome the "clicking". I tried to isolate it to the tank hose and did that by changing the two hoses (leaving the tanks where they are located. That eliminated the clicking with the left tank, it moved with the hose to the right tank. To date, I haven't changed out the "sensitive hose" (it still works and is not a safety issue) more because I keep forgetting to buy a replacement rather than because of a "need to eliminate the clicking"...

If I were you, I'd first put the bottles on the opposite hoses, see if the humming continues. If it does, I'd replace the regulator first, hoses second, or to "kill two birds with one stone" just buy two hoses and a regulator assembly and replace the entire thing.... SAVE THE OLD SYSTEM AS AN EMERGENCY SPARE..... You never know when a regulator or hose will spring a leak and/or stop working.....