Dust on the fan could have it out of balance… open it up and clean it… while you have it apart… look for loose nuts the hold the light housing to the fan housing… this can rattle loose. Its probably not dust on the fan, but the process of taking it apart to clean the dust, and putting it back together, could fix your rattle. Take one part off… turn the fan on… does it still rattle? Take another part off… turn the fan on… does it still rattle. If you get it apart where the only thing left is the FAN itself… well…
Could be the exhaust vent pipe rattling against the fan housing… go in the attic and use SILVER TAPE for ducts to sure up the connection.
Are the springs solid on the fan cover, is that rattling around?
Once you find the source, silver-tape can help keep things from moving around. I’ve jammed rubber from an old bike tire inner tube into spaces to prevent metal to metal contact…
Take it apart… you can’t make it any worse… and if you break it… you can hire one of us to install a new one for you.