Is the "pop" you refer to a coil pop like it is trying to eject a ball? If so, there is a stuck switch that causes the machine to think there is a ball somewhere that needs to be ejected. Note: You have a "System 9" game.
Below is an explanation of how to put your machine into diagnostic mode to check for a stuck switch and here is where it came from:
http://www.pinwiki.com/wiki/index.php?title=Williams_System_9_-_11 Hope that gets you giong in the right direction. Remember it not only could be switch contacts too close, worn out or broken, but also could be broken and shorted wires in there as well. Hopefully you will find the problem before having to troubleshoot the boards. Good luck.
3.9 Accessing Bookkeeping, Settings, and Diagnostic Modes
The coin door buttons for system 9 and 11 games are shown above. The function of the three buttons is (from left to right in the picture)...
Reset High Score to Date
Auto Up/Manual Down
Advance
Function of the Reset High Score to Date is self explanatory.
The Auto Up/Manual Down (AUMD) button is interpreted by the game software in multiple ways. Starting from attract mode, if the AUMD switch is up, pressing the advance button causes the game to enter audits and adjustments mode. If the AUMD switch is down, pressing the advance button causes the game to enter diagnostic mode. With the AUMD button in the up position during many of the diagnostic tests, the next step in the test will be automatically entered. For example, display test will display each digit on the display in turn. Coil test will advance through each coil. A particular step of the test may be paused on by pressing the AUMD button down.
The Advance switch moves the diagnostic, audit, or adjustment to the next step (or previous step if the AUMD button is down, except in diagnostic mode).
When in audit or adjustments mode, the credit button is used to zero, turn off, or turn on features.
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