after fast accl. Car now has a faint flapping/metal knock 40mph+ why?

What is your question?

I was merging onto a highway and accelerated rather quickly , almost instantly I heard a sound that sounded like wind flapping a piece of paper in the wind , but louder. I can still hear the sound when driving st speed of 40mph+ with my windows completely up. Occasionally there is a faint metal knocking sound at these speeds also. When driving in the city when I push the gas a little bit and then let off of it my car has a tiny tiny jerk to it, what's going on???? :(((