Peugeot 1.8 CAB grinding noise possibly exhaust related.

There is a loud grinding noise coming from underneath my feet when accelerating, especially from a stop,
It cannot be heard from outside the vehicle but it sounds as if something is grinding away and I hesitate to accelerate because of it so im driving round (i have to get to work daily) very very slowly and gently!
Could this be exhaust related and if it was a blowing exhaust why doesnt it make this noise when stopped and I put my foot down on the pedal? And why does it sound more like metal grinding (from inside only which can be deceiving ofcourse) than a loud blowing exhaust?
any help greatly appreciated! :)