Here's some more input from the homeowners- noise is definitely sharp cracking. They have watched the timber checks and are confident that the timbers are stable now.
They are sure the noises are coming from several spots in the roof but can't locate the noise, probably because the entire house is one full cathedral ceiling and sound bounces around everywhere. They don't think there are any moisture issues.

They are asking if adding more screws from panels to timbers will fix the problem? Has anyone tried this before?