What was found
(CAN) DURING A MAINTENANCE GROUND RUN, AND FOLLOWING THE START OF THE RT ENGINE, A HISSING SOUND WAS NOTICED EMITTING FROM BEHIND THE INSTRUMENT PANEL. AFTER FURTHER INVESTIGATION IF WAS FOUND THAT THE INSTRUMENT AIR TUBE HAD BEEN CHAFING ON THE UPPER CORNER OF THE RADIO RACK. THE TUBE IS USED TO JOIN THE VACUUM REGULATOR TO THE LT SIDE OF THE FORWARD BULKHEAD TO THE MANIFOLD LOCATED IN THE COCKPIT BEHIND THE INSTRUMENT PANEL ON THE RT SIDE.