What was found
LOG PAGE 8429793 - FLT 771 CLT-SFO - RETURN TO FIELD - SHORTLY AFTER DEPARTURE, CREW SENT ACARS MESSAGE INDICATING THEY HEARD POPPING AND GRINDING NOISES DIRECTLY BELOW THE COCKPIT AND THOUGHT THEY HAD LOST THE NOSE GEAR. . PASSENGERS REPORTED HEARING BANGS AND FLASHES FROM THE NO. 1 ENGINE. AN EMERGENCY WAS DECLARED AND THE AIRCRAFT RETURNED TO CLT LANDING WITHOUT INCIDENT. MAINTENANCE PERFORMED EXTENSION/RETRACTION CHECK OF NOSE LANDING GEAR WITH NO FAULTS FOUND. ACCOMPLISHED BORESCOPE OF NO. 1 ENGINE AND REPLACED THE HYDRO MECHANICAL UNIT (HMU). ALL OPERATIONAL CHECKS WERE GOOD.