Your muffler is either blown, has a loose connection, or you have a broken exhaust pipe between the exhaust manifold and the muffler.
Loud Muffler Sound When Starting The Car, Can You Help?
My car use to do that I had to replace the pipe from the cat all the way down to the muffler and I had to replace the muffler as well. After I replaced it I started my car and I couldn't even hear it run it was great.