MD83 Captain is informed of pieces of metal being found by Maintenance when the cowl is opened for inspection. Where the metal pieces came from is never discovered but the aircraft is released for flight.

Narrative

I arrived at aircraft and Maintenance was working on the left engine and said that they had found these pieces of metal (2 large pieces and 3 or 4 smaller pieces of what appeared to be a very large washer approximately 5-6 inches in diameter) laying inside the engine cowling when it was opened. They inspected the lower part of the engine but found nothing out of place. Two mechanics were working on the issue and one of the mechanics told me that he thought that the metal may have come from one of the engine bleed valves; as he had experience changing valves. A supervisor arrives; looks at the metal and says we have to figure where the metal came from and that they are taking the jet out of service. The supervisor placed the metal parts in the aircraft logbook. On a side note; two flight attendants were on board when Maintenance did an engine run at the gate before I arrived and made it a point to let me know that when Maintenance ran the engine there was a 'gurgling and banging noise' coming from the engine. I passed this information onto both mechanics and the Supervisor. I just checked out of curiosity to see what Maintenance found: DURING T/S AT LH ENG OPENING COWLING AND FOUND SEVERAL FLAT PIECES OF MASTER (I think MASTER is supposed to be METAL) IN LWR COWLING SUSPECT FROM UNDER ENG BLEED VLVS PIECES ARE IN LOG BOOK. INSPECTED ENGINE UNABLE TO FIND ANY DAMAGE OR MISSING PARTS.I saw the parts; they came from somewhere off the engine with enough force to break in to multiple pieces! You don't find metal pieces just lying in the engine cowling; where did they come from? I don't understand how this aircraft was released into service without a positive fix or finding of where this metal came from.