Loud, rhythmic mechanical/rotational noise from underneath when braking & coming to stop--not tire noise

Subaru Forester xt--not turbo. Noise began at about 55,000 mi. Very loud, unreal, rotational noise when braking and coming to a stop at slow speeds. First began sporadically and now very consistent, and getting louder. Last time at Subaru dealership for maintenance could not get it to duplicate noise. Yesterday, tire and brake specialists examined tires & brakes and heard noise when braking but have ruled out tires or brakes, both of which are relatively new and functioning ok. TX fluid ok. Honestly, it sounds like the undercarriage of the car is going to fall off when I step on brake pedal. They are suspecting problem is in drivetrain.