I've discovered that the gap between valve covers 1 and 3 is .005 in. (.13 mm), and the gap between 2 and 4 is basically zero (too small to measure).  I do not know if these gaps ever met the specified minimum of .008 in.  There has been no major service to the engine since new.  The engine is a 912ULS, 2.5 year old, with 300 hours. 

Clearly this needs to be corrected.  What I want to know is:

1.  Assuming these were correct when the engine was new, why would the gap shrink over time?

2.  What damage could result from running in this condition?

3.  What audible NOISES might result from this condition? 

I found the above condition while searching for the source of a noise (almost like knocking), which is why I ask about the kinds of noises that might result from that too-small gap, or possibly some other condition that is causing the gap to shrink while also producing noise. 

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