I had the same sound on my 06 Newmar Kountry Star. I'm on a Workhorse chassis and you're on a Ford so this may or may not be of any help. After much searching I discovered two "small" broken welds on the passenger side where the "front cap" was attached to the frame. It was a relatively easy and cheap fix once I discovered the problem. The other symptom I had other than the popping noise, which I usually heard at low speeds when turning, was the corners of my windshield were popping out of the gasket. I had them reset twice before I discovered the defective welds. I might never of found them if someone on this forum told me look for them because they had the same problem. Good luck finding the cause.
Bob & Leah, USN Retired
2016 Itasca Suncruiser 38Q/SOLD
2014 Chevy Equinox LTZ/Blue Ox Tow Bar