I have a Savinelli Pisa 114 KS that has a small fill on the outside of the heel. Every time I smoke it (I think) it makes a popping/cracking sound and has a feel to it not unlike holding a glass of ice and filling it with warm water and slightly cracking the ice. I think it's the fill, but I'm not sure. It doesn't really bother me and it smokes fine. I just never knew what it was. It's the only pipe of mine that does this and I noticed this 'pop' before I even noticed the fill. Probably from inspecting the bowl for cracks, which is what I initially thought was happening from smoking too hot. Thoughts?