I think part of it has to do with the car gaining or losing connection to HD radio due to slight interruptions in signal strength. I've noticed this along with an unusual stutter or heterodyne sound. It sounds like the radio also filters out static, it's very "all or nothing." I've never had an HD receiver in a car so it's not what I'm used to hearing. My old Altima had a conformal antenna built into the rear window and the radio was very clear relative to the shark fin antenna we have.
Mostly I've just been listening to Sirius and Spotify, but I tune in to AM for sports and FM for NPR. NPR stations are almost always at the bottom of the frequency range where you're more likely to get interference already, so I lose the signal constantly.