What was found
FLT 1131, AIR TURN BACK, EMERGENCY DECLARED. CREW EXPERIENCED A BURNING PLASTIC ODOR IN THE CABIN. PAX REPORTED THERE WAS POP ON THE RT SIDE OF THE ACFT RIGHT BEFORE THE ODOR AND SMOKE. PAX ALSO REPORTED THAT GRAY PEBBLES WERE SHOOTING OUT OF THE OVERHEAD VENTS AT THE SAME TIME AS WELL. ODOR WAS REPORTED AS EMITTING FROM THE GASPER VENTS. EMERGENCY WAS DECLARED AND THE AIRCRAFT LANDED WITHOUT INCIDENT. DURING INSPECTION, NR 2 ACM WAS FOUND BURNED OUT. R & R ACM. PERFORMED OPS CHECK. OPS CHECK PASSED.