I would suspect that what you are hearing is as pointed out above. Wood which is used for floorboards expands and contracts depending on the humidity. We have one floorboard in the bedroom of our 5er that moves that way. We don't like it, but it is a quirk of ours.
Another point - ours had a light jingling noise particularly when we walked around the LR/kitchen. I spent virtually hours snugging fasteners anywhere found. I replaced the original dry, worn out spring shackles with wet bolts this spring at about 15,000 miles. No more jingles
Dave W along with my DW, Susan and our poodlepups, Callie & Molly,2011 Ford F250 6.7 CCLB, 5er Hitch Option w/B&W Hitch,,Ride Rite air bags, 2014 Montana High Country 343RL (38')