Expansion or contraction of framing members. Tension builds until it is able to slip and "pop" it releases the tension. I have one in the ceiling / attic above my bedroom that pops on occasion when there is a gusty breeze or just the sun coming up and rapidly heats the decking after a cool night.

Short of rebuilding, I don't know how you would track it down or fix it.

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Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002