What was found
AFTER APU START GROUND AGENTS NOTICED MIST LIKE SUBSTANCE COMING FROM FO SIDE RAM AIR PORT. REMOVED FROM SERVICE. RAN UP APU AGAIN, NO SUBSTANCE3 WAS NOTICED IN THE AREA OF RAM AIR PORT. NO FURTHER DEFECTS. PACK WAS ON MEL 21-51-00A. VERIFIED FAULT CODE 21515700AM1 AND CONTINUED FIM TASK 21-51-00-810-802 REPLACED AIR CYCLE MACHINE BEAN RR OF #1 ACM REF AMM TASK 21-51-05-000-801-A AND 21-51-05-400-801-A STEPS 1-9 B. UPON REMOVEAL ACM FOUND SEIZED TOOK AT LEAST 220 INLBS OF FORCE TO BREAK FREE. CONTINUED INSTALLATION OF # 1 ACM REF E 175 AMM TASK 21-51-05-400-801-A PARA 9C-F CLEARED MEL 51-51-00(A). RETURNED TO SERVICE.