What was found
INFLIGHT (BOTH ENGINES ON) AND BOTH PACKS OPERATING, UNUSUALLY LOUD MAKING INTERCOM AND ATC EXTREMELY DIFFICULT, NOISE REDUCTION SIGNIFICANTLY REDUCED WHEN 1 PACK SELECTED OFF. SQUEALS NOTED W/ BOTH PACKS. PERFORMED ENGINE RUN, PACK OPERATION TO DETERMINE NOISE, PACK OPERATIONAL TEST SATISFACTORY WITH NO ISSUES NOTED, IAW B767 AMM 21-00-00. PERFORMED PNEUMATIC SYSTEMS OPERATIONAL TEST, PER AMM 36-00-00. DUCT PRESSURE NORMAL. COULD NOT DUPLICATE DISCREPANCY. OPERATIONS/LEAK CHECK GOOD AT THIS TIME.