This sounds exactly what mine was doing and all mine was due to low air pressure. They were at 25 psi, aired them to 35 and the sound is almost all gone.
Now it very, very faint so I am pretty sure it is just tire-rubber related. These tires still have plenty of tread left but after reading reviews others have stated that the rubber becomes hard at noisy. The tires are the barum bravuris and will be my last set.
Hope this helps.....