You probably have surface difference side to side. The contact face, rotor surface to pad surface, is an eternally changing system. Pad material is burned, mixed, diffused into the surface of rotors changing the friction coefficient of each surface. Certain combinations (of surfaces) cause various vibrations of the system.
These vibrations manifest themselves as, squeaks, moans, and the condition called "crouching' (first described that way to me by a MB factory rep in 1975). This last vibration condition isn't heard but felt and it feels like the system is metal to metal (best way to describe it).
The condition almost always can be removed by sanding with 80 grit sandpaper (all surfaces). While this will get immediate results they may not be permanent. Pad composition eventually is the long term solution. (along with either new rotors or a thorough sanding) MB has different pad compositions for different noises.
BTW none of this usually affects the brakes stopping ability. It is only a creature comfort issue. IMHO.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician