2008 Ford F-150, RWD

Today when driving to lunch I noticed a subtle humming or buzzing kind of sound, sort of like a strain of some kind, while braking and accelerating from a stop.

After driving about 5 minutes when the truck was warmed up, I couldn't hear it anymore. Turned it off for a few minutes, started it back up and the noise was there again for another couple minutes before going away.

Any ideas what this might be? Got the truck fairly recently so no real history with an issue like this. Thanks.