The sound you are hearing is most likely the metal stove pipe heating up and expanding, which is normal.
If you have concerns please call a local stove shop or chimney place and have them take a lot at it.
Better safe than sorry

It's probably the metal expanding and nothing to worry about much but there's a chance that some of the sounds are from falling creosote particles from the lower flue temperature. By Heritage, I am assuming you mean a Hearthstone Heritage, nice stove, but 325 is pretty low for your flue temperature if you're using a magnetic type thermometer. I used to have a Hearthstone Phoenix and cruising temperature on that was between 400 and 500 for most of the burn.

How far up the pipe is your thermometer from the stove flue collar? It might read lower if more than 2 feet above.