My WAG is one or more pads is dragging against the rotor. In my experience: cars before my Hellcat, my Hellcat, and more recently my Scat Pack and even my M-B cargo van; this can and does lead to faint intermittent low speed brake squeak or in some cases a rubbing noise.
My solution is to wash the brakes at a DIY car wash. No need to jam the wand nozzle down into the area of the pistons. The jet spray could tear the brake piston dust boots.
Just a thorough spray down on the soap setting followed by a through rinse then finish with a good rinsing using the no spot rinse setting.
Afterwards be sure you drive the car enough to use the brakes and hard enough to thoroughly dry the brake hardware. This includes the parking brake hardware "buried" in the rear hubs.
If you use plain water setting at first you can see how much dust come off the wheel/brake hardware. The water runs black.
(The dust build up on the dust boots interferes with the natural tendency of the piston seals to cause the piston to slightly retract when the brake pedal is released. This moves the pad just far enough away it no longer rubs against the rotor. If it doesn't retract... well I think you are hearing what this can mean.)
On a related note in my Hellcat shortly after I pulled out of a restaurant parking lot I heard a thumping from my Hellcat. Pulled into another handy parking lot and checked and didn't see anything.
Drove on. Got on the freeway and heard the noise again. It was clearly related to speed.
Took the next exit which was my exit and the short distance home. No other symptoms other than just the thumping noise.
Once home in the drive I parked car and got out and looked again. This time I spotted a length of apparently road tar that the right front tire had managed to pick up as I left the restaurant parking lot and drove though a bit of road work area with some repaving going on. I didn't drive over fresh paved or tarred road.
What I picked up apparently was a length of tar that resulted from a leaking valve and the tar tanker truck being moved which laid down a fraction of an inch diameter and who knows how long a strand of tar. The piece that was stuck to the tire -- in one of the circumferential grooves -- was approx. 18" long. It was stuck to the tire well enough that even I'm sure over 50mph it did not get thrown off. Never had that happen before.
