Citroen C3 rasping throaty, exhaust -type sound coming from engine bay.

There is a noise coming from the engine bay that sounds very much like a blowing exhaust. It gets louder on engine load and speed increase, but is not really audible at idle. Engine has approx.163, 000 miles. Vibration can be felt in passenger and driver footwells on speed increase also. Inspection of Manifold/ cat pipe/exhaust pipe blowing has found nothing, and a new exhaust manifold gasket was fitted thinking it may have solved the problem. I am a bit lost as to what it might be. There are no other sounds of "impending catastrophy", and the car drives fine other than the noise symptoms mentioned. Exhaust back pressure seems a bit low - but I wouldn't know if that is normal. Please help..