What was found
DURING CRUISE FLT @ FL 190, 7.5 PSI, A HUMMING NOISE ACCOMPLIED BY A VIBRATION WAS HEARD AND FELT. LOUDEST SPOT WAS LEFT SIDE OF AIRCRAFT IN THE FRONT HALF OF THE CABIN. AT TOP OF DESCENT AS THE THROTTLES REDUCED, SO DID THE NOISE & VIBRATION. AT LOW THROTTLE SETTINGS & THEN AT LOW ATITUDES THE HUM & VIBRATION WERE GONE. ENGINE VIBRATION WAS NORMAL IN ALL SITUATIONS. REMOVED FROM SERVICE. REF WO 352421, N/R-00001 FOR WORK PERFORMED AND REPAIR OF CRACKED R/H PACK RAM AIR INLET DUCT. COOLING PACK OPS TEST GOOD. NO VIBRATIONS FELT. RETURNED TO SERVICE.