When the LT5 was under development there was an odd spark plug failure mode. The center electrode's ceramic insulator would break off and slide down the center electrode. Lotus engineers would call a spark plug that failed in this way a "rattler". When shaken by hand, the ceramic piece slides up and down on the electrode and makes the rattling sound. The problem was addressed and the spark plug was improved. However, at a very low rate, the failure can still occur. We found this OE FR2LS rattler in '92 VIN 215 with about 20k miles. I encountered another rattler in another engine several years ago. The FR2LS copper spark plug was discontinued by AC-Delco in about 2006. It was replaced with the 41-602 copper spark plug.