Feeder problem?

Hey I'm running a Heidelberg sm 52. Ever so often when I start feeding paper I will hear a clunk sound as it gets ready to feed. Trying to figure out what it is so I can fix it.

 

What you are likely hearing is a valve that is opening or closing under the feed board. This is operated by a small pneumatic cylinder under the belt. This can be easily seen if you remove all paper from the feeder and remove the metal plate behind the pile stop rails. This valve is in place to help with the final sheets going down the feed board as the belt will lose vacuum due to lack of sheets. It is possible that the flap that opens and closes has detached from the shaft. You can try simply unplugging the pneumatic cylinder and see if this solves your problem. If you do this you may find that the press will not want to take the last sheet or two into the press. I have unplugged this cylinder before and never had an issue.