What's The knocking?

I was driving a State pool car since mine was in the shop. A girl named Cindy and I were returning back to Baltimore from the Eastern Shore of Maryland following a nite time public hearing.

On a particularly dark and lonely stretch of road, a knocking noise started coming from the engine. "That's odd." I said, "I wonder what that knocking is?"

"Well, I can tell you one thing for sure." Cindy answered icily. "It sure ain't opportunity."