What was found
AIRCRAFT WAS GROUNDED: RETURNED TO DFW ON ACCOUNT OF LOW FREQUENCY VIBRATION/RUMBLE AT ALL SPEEDS, APPEARED/FELT TO BE EMANATING FROM FORWARD PORTION OF AIRCRAFT. RUMBLE DID NOT CHANGE WITH POWER CHANGES, GEAR EXTENSION, OR FLAP EXTENSION, RUMBLE CONTINUED, MUCH REDUCED, ON TAXI IN TO GATE. EMERGENCY WAS DECLARED. FOUND ANTENNA DRIVE UNIT MAKING SEVERE GRINDING NOISE WHILE SWEEPING. REPLACED WXR DRIVE UNIT REF MM 34-43-11, OPS CHECKS GOOD.