How about a wheel (rim) barely rubbing against a slightly bent [brake] backing plate? Could just be a slightly warped rotor rubbing the brake pads (or a worn ridge on a rotor).

Another thought, I just did the brakes on my W124. I changed out the rear rotors and I was surprised to see the poor condition of the parking brakes. They were very rusted and with large pieces of brake lining having fallen off and rubbing against the spinning rotor. Mine didn't make any noise that I noticed, but I could easily see where such a situation could make some low growling, grating sounds.

A stretch I know, but just some things coming to mind that might not otherwise be considered! Obviously there's a laundry list of rotating, moving parts along the entire drive train that could produce hard-to-pin-point noises.