I had a noise something like that last winter. After several bottle changes. the noise was still there. I could not imagine anything that could possibly go wrong with a pigtail that could noises like that. By process of elimination, I concluded that there must be something wrong with the regulator.
I disconnected the pigtail from the regulator, and a significant amount of oil ran out of both the regulator and the pigtail. I reassembled, and the noise was gone. I have come to believe that the noise was propane bubbling through the oil.
2011 Silverado 2500HD Duramax, 4x4, crew cab, long bed
Palomino Puma 253-FBS, 27' 5th wheel
1994 19' Class B on Chevy chassis