During the downwind leg of the circuit, the pilot heard a sharp cracking noise, which was accompanied by a vibration in the airframe. He also noted that the transmission oil pressure warning light had illuminated. A precautionary landing was carried out, and an initial inspection revealed that a transmission cowling had become detached in flight. The cowling had struck the main rotor disc, and a piece of debris had then hit a tail rotor blade. The design of the latches of the cowling is such that they can appear to be locked when they are actually in an unsecured position. It was possible that the pilot had not fully secured the cowling during his pre-flight inspection. However, the locks were not recovered and the precise reason for the cowling opening in flight could not be established. A detailed examination failed to reveal any reason for the illumination of the oil pressure warning light. It was likely that the warning was spurious, probably being generated by moisture around the pressure sensing switch wiring.