I have a Poulan push mower that after mowing in a ditch started making a loud noise and running rough. It almost sounded as if metal was hitting metal. The mower was vibrating more than normal. I checked under the mower and nothing is hitting. After a few times of stopping the mower engine because it sound as if it is going to die down the mower will not start now. The shaft does not appear bent and the blade appears ok. The mower has oil, a good spark plug and a clean air filter. It was running fine until that moment. I did not see anything that I hit but it is possible I hit something in the ditch like a pine cone or branch. What could the problem be and how would I go about fixing it?