there is definitely hissing, but I don't think that it is high pitched. Actually, I just got the gas fireplace a few months ago and after I would turn it off I would hear what I thought sounded like a gas leak in the house...this was before I realized that I had T (my T started 3 weeks ago). Now that I hear the T consistently I realize that the noise I heard once the fireplace was turned off was the T and not a sound in my house. I wonder if the gas fireplace caused the T.