TBH, the Mk2 is no worse than the 2003 on Mk1- Project Drive had already seen the deletion of all the sound insulation that was removed by then, and whilst the difference in noise levels between the pre- Project Drive cars and those that had some of the sound insulation deleted is certainly noticeable, it isn't actually a massive difference.
Also, the ZR is noisy compared with the 25 deliberately - a larger bore exhaust than the equivalent Rover 25 gives them a throatier, more sporty sound.
The L series diesel engine is quite a noisy engine compared with the K series petrol, and the exhaust system fitted can make a huge difference to the noise level in the passenger compartment. Many have been fitted with deliberately noisy aftermarket exhaust systems by (mainly youngish) owners who seem to think noise = power. I would check what system is installed on yours - if fitted with one of those noisy 'performance' systems, a return to a standard exhaust system may see a very big reduction in cabin noise.