Horrendous exhaust cabin noise

I have a 2001 330i tourer. The exhaust sounds great driving around town but on the motorway it's horrendous. The noise in the cabin gives me a headache. We went to the lake district yesterday, all our ears were hurting when we came off the motorway and then the revs increased going up all the hills and the noise got louder than on the motorway, it was just horrible, I've never driven a car that's so loud in the cabin. My son did complain that his ears hurt. Does anyone else have this kind of exhaust cabin noise, is it normal? I'm wondering if the previous owner has done something to the exhaust or does the standard system make loud cabin noise? My car has M Sport suspension so it could have an M Sport exhaust, would that make the noise I'm talking about? Is there anyway to stop the horrendous cabin noise or at least greatly reduce it?