This this out.
My 2004 bifuel regulator
This might be the problem for the pops/swooshing sounds. Just FYI, I emailed GFI a while ago and asked if they might have some input given the difficultly we've had in having a shop track down the problem.
I did find a replacement regulator (p/n
3Z-9C968-AB)
2002 Ford F150 - 1921517 Compuvalve vapor assembly - F150 - CNG/GAS