For those of you with flex liners, do you notice that sometimes the liner makes dull "thud" sound? I noticed today (really cold out) that I hear 1 dull bang/thud from in the chimney. The stove has been going all weekend. I have a brick chimney with clay flue with the SS liner inside that. My stove store said that this is normal as the flex liner tends to expand when heated and cooled. What do you guys think?