I'm not sure I am understanding this. Are you saying you are burning ceramic logs in a gas fireplace without an opened damper? OR are you saying it is wide open all the way up? What do you mean seal up the chimney?

To answer your question...My fireplace is or was woodburning. I hate messing with wood, so I had a full burner installed and use it with the ceramic logs(with all the phoney ashes that we work so hard to keep cleaned up when they are real ashes). I do NOT hear any hissing. I'd ask a technician at the fireplace retail store. It doesn't sound right to me.

Yes, I am burning ceramic logs with natural gas. They are ventless, so I do not need an open damper. I've been told I should put a plate over the chimney to stop downdraft and/or critters & birds from nesting in it as the chimney serves no purpose. Also, it has no damper - its a straight shot to the great outdoors. The sound it is making reminds me of a gas grill when you turn the propane tank on. I think you are right I need to have someone look at it. Unfortunately, I had someone who could "get you the logs and install them" do the work. Not exactly a real dealer.