exhaust system ford mondeo 1998?

I have a 1998 ford mondeo estate and I'm currently experiecing difficulties with it running. I have narrowed the problem down to the exhaust system which when accelerating occurs the exhaust "put,puts"!!. engine runs smoothly on idle an rev counter just below the 1000 mark. No over heating. Seems to be middle box coughing but not leaking. Any help greatly recieved.