Pontiac: back up noise, fog horn, rear brakes

Hello Harold: This may be very difficult to answer..but my daughter, who has a 1995 Pontiac Grand Am says every so often when she backs up she hears a noise that she describes as
"like a foghorn". She worries about her transmission..i have checked the fluid (a little dark but no burnt smell).
Make any snese to an expert?
thanks very much
bob miller

I haven't heard any fog horn noises lately but I doubt that it is from the trans. If she only hears it when she first backs up after the car has been sitting all day or night, it could just be noise from the rear brakes. It could also be an exhaust seal that is going bad and when the engine is torqued in reverse the seal opens up.
Hope this helps.