What was found
EN ROUTE TO PHX, ONCE AGAIN EXPERIENCED A VIBRATION. IN PHX ADDITIONAL CHECKS WERE PERFORMED TO THE RUDDER SYSTEM; THE STANDBY RUDDER FAIL AMM TEST 27-21-141 STEP 2B; THE STANDBY RUDDER DEAD BAND WAS ADJUSTED IN LIMITS PER AMM 27-21-14-1. THE PROBLEM CAUSING THE VIBRATION WAS ISOLATED TO THE LT AIR CONDITIONING HEAT EXCHANGER EXHAUST DUCT WHICH WAS BADLY DETERIORATED. REPLACED DUCT AND NO FURTHER PROBLEMS HAVE BEEN REPORTED.