Hi guy`s, my disco td5 2001 groans as well, I asked the garage where i got it and also an independant Landrover specialist and they both said the same, apparantly the pipes are harnessed to the body work and due to the pressure in the pipes this echoes/vibrates areas of the body/chassis which makes it sound really loud, i kind of settled with this explanation and put up with it. Not sure tho.