Occasionally you will hit something hard enough to bend the deck or the blade. With such thin metal decks today, they are easily bent. If your deck is rusting, parts can come loose and hit the blade as well. Some decks have guards inside of them that are spot welded together and when those welds come loose, the guard will hit the mower blade and you will hear quite the racket!
To inspect this area, pull the spark plug boot off the spark plug and then raise the mower deck from the front. It is not a good idea to lean the mower over side to side as this can cause the oil to leak out of the carburetor through the air cleaner.
Look for anything that is loose or for the blade that is bent. Slowly rotate the blade to see if it hits anything and you will likely find the cause of this problem pretty quick. Either the blade will need to be replaced or the deck metal repaired. You should also check to see if the crankshaft is bent while you are slowly turning the blade as this can cause the problem as well.
If you need a lawn mower repair shop to check out this problem for you, visit our Locations page for help finding a small engine shop in your area.