Could be a bad bearing, but I cant remember for the life of me what turtles banging sound like again, care you try and type it out so I can get a better understanding? Haha all jokes aside try checking your bearings, also do you have your drag cranked down all the way and how tight is your spool tension knob. You could also check the little red/green brake pins, make sure they are all still in place. If your not comfortable taking your reel apart maybe have it sent out. There are loads of guys here who do service reels.
Update... dont google turtles banging ?