Can the noise from the external exhaust pipes of a gas furnace be muffled? The noise is loud and disturbing especially at night?

I wouldn`t muffle,, but you could add to it and get it higher in the air where the sound won`t be as close to home or windows

Mine is very quiet. Are you sure you don't have water in the exhaust side? It makes a gurgling sound. Otherwise, add extensions to the piping and run them around a corner.