In the picture of the back before applying linen, it looks like cambium remnants are left on the back. Sometimes these want to pop off/and or lift. They don't pose any problem, but should be scraped off. That could potentially account for small amounts of cracking/creaking sounds. Hope it is nothing major.
From the belly, it looks like maybe a high crowned stave from a small diam. tree, but pictures of back look relatively flat backed. If the crown is high, maybe consider decrowning.