Usually it means something is loose. If it happened right after you did the power steering, then get back under and check everything to make sure it's all tight and fastened. You might have had to loosen something to get to power steering, and forgot. But it seems unlikely that a loose piece under the hood would cause a loud bang.
SEriously, do you have a loose bowling ball in trunk? Is there something you've got in trunk that is loose and moving when you turn? It's not so stupid a question. It might be something that came loose or that you never thot would move.