1997 Other Mazda Models Exhaust pipe

Noises problem
1997 Other Mazda Models 4 cyl Manual 90k miles

My car is making a vibrating noise coming from the exhaust pipe, not from the end of it, but the all thing makes this vibrating noise. I first thought it was shaking and hitting on something, but I went under the car to have a look with the car on and off, and I can shake the pipe ( when off) to make the noise which actually sounds like could be something "rolling" inside the pipe, about in the centre of the car not towards the end of the pipe. Do I need to change the pipe? It is a Mazda 121