As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
COULD HEAR A LIGHT RUMBLING SOUND IN CLIMB CRUISE & DESCENT. NOTICED THAT THE RUDDER WAS FLUCTUATING LT & RT WITHIN THE RUDDER LIMITS, NO YAW WAS NOTICED. FLUCTUATION REDUCED AT LOWER SPEEDS. ACCOMPLISHED TROUBLESHOOTING MANUAL TASK 21-26-00-810-837-A FOR RUMBLING NOISE IN COCKPIT, NO DEFECTS NOTED. ALSO TROUBLESHOOTING MANUAL TASK 27-20-00-810-806-A FOR RUDDER FLUCTUATING. FOUND SYS DATA ACQUISITION CONCENTRATOR 1 CAUSING RUDDER TO FLUCTUATE ON FLIGHT CONTROL PAGE. REPLACED SYS DATA ACQUISITION CONCENTRATOR 1 IAW AMM 31-55-34. OPERATIONS CHECKED GOOD.