most likely just a vibration. i'd focus on the fronts first, though you can do whatever you'd like. any small adjustment can change the harmonic creating the issue, so you can swap pads from left to right. if you havne't put many miles on them you should be good to go. Duralast is as basic as you get, so you should have much in the form of pad transfer to deal with. if front doesn't work you can do the same to the rears.

you can troubleshoot the issue if you can easily reproduce the sound at low speed with a friend outside and you jogging next to the car. be happy at least everything feels good though, so it's more an annoying thing than a safety concern.

 

Along with what tea said. A positive only test would be to pull the e-brake; if you hear noise when you do it then you would know it's the rear (if you dont here anything then that does mean it's not the rear).

Before you do anything youve done the bed in right?