Excessive Engine / Exhuast Noise at 70mph

[ali]

I've a P reg '96 Laguna Rti 2.0 Sport.
Only had the car about 3 months, but I find that when I reach 70mph engine / exhaust noise rises's dramatically. from 80mph onwards it's quietens comparitively to the speed of the engine.
Back and middle sections have recently been replaced due to knackered baffels but this sounds remains. Garage inform's front pipes / cat ok.
Anyone any ideas. It seems to be a resonating sound so maybe somethings not quiet tight enough.

[MC]

Could be an exhaust shield - look at the one on the front pipe. It could be a cracked front pipe under the manifold - very hard if not impossible to see as the heat shield covers the area which can crack - may need removal to inspect.
Good luck!
MC