Thats a tough one, but that sound probably isn't good. If your O/D light is constantly on or flashing get the tranny to a shop to run a diagnostic because it is probably about ready to die.
Make sure your O/D button on the shifter is working right to make sure the light is on for another reason. When my OD light flashed at me and then stayed on, but went off. I ended up getting a new tranny.
