I have 2 that do the ping at the 45 minute point, too. It seems to be caused by the hammer lever slipping off the 'stop' arm as the 2 pin wheel rotates. (Apologies for lay-terms). On another movement the end of the stop arm is slightly notched, which seems to hold the hammer until the hour sequence starts, on these 2 it's sloped. I'm tempted to take a dremel to the 2 and make notches - anyone got a more sensible answer?