I have a new Accentra 52i insert and it works great so far as far as throwing heat. The issue is it makes a low rumbling- flutter sound frequently when running. It sounds almost like a delivery truck is idling down the street. I had it professionally installed in an existing masonry fireplace. The rumbling gets much louder when the hopper lid is open. Any idea what is causing this sound and is it normal? Thanks for the input.