Peter,

I have a North Americian 242 (which were ususally optioned differently, and had different emissions requirements) but some of the following may apply:

1) Engine Fan - on all pre-'81 240s (at least US), the engine fan did not use a clutch to reduce engine fan speed at higher RPMs.. which gives the car that "Volvo Roar" that you're hearing. Updating to a clutched fan assembly or an electric fan is said to help a good deal. (I'm updating to a Volvo electric fan found on later 7/9 series cars, but there are lots of solutions for this).

2) Axle ratio - most of the older 240s had high-ratio (low geared) rear axles, usually 3.91:1 - which makes cruising above 60mph/100km noisy. Swapping the axle for a later one can reduce the revs at speed.. but you lose low end pull. This isn't fun, either.. (and is something I'm trying to avoid doing).

3) More insulation - yes, later cars had better insulation, but if you're going to start adding go straight to the aftermarket. ipd offers a hood insulation kit that reduces the interior noise levels by 2-3 db, and numerous companies offer solutions on making the car quieter (spray on dampeners, more effective pad insulation, etc).

I'm not sure about what's available in your neck of the woods, but go down and talk to the people at a high end car audio store and they'll be able to steer you in the right direction.

Good Luck! - Erston