I haven't seen any technical service bulletins, but incorrectly installed
windshield could be the cause. I worked on one of these recently, and
just before I was ready to yank the dash out, the little light bulb in my
head went off and I removed the outside windshield cowling, and presto,
the noise went away.
What I eventually found was that one of the tabs on the cowling was
pinched between the windshield and the body, causing a light popping noise
when driving over bumps. I cut off the one tab in question and trimmed the
others close by to be sure they wouldn't get pinched, either. Upon
reassembly, there was no more problem.