Most of the big inline-6 engines in the 960/S90/V90 and their 5-cylinder cousins in the 850/S70/V70 exhibit some valve sound at startup. There's been a lot of discussion about it here. In my experience, there's no negative effect as long as the noise essentially goes away when the engine is fully warmed up. Some cars take as long as 15 minutes to quiet. Some cars are much more affected by disuse than others. Just about all of them sound terrible if allowed to sit for a week. There's a real clatter from the valvetrain.
I drove a V70 recently that refused to be quiet until we drove it for an hour on the highway- it had sat for a month after being sold by Volvo Finance (a lease car). It sounded terrible. The owner was calling around for new engines... and after one 50-mile highway drive, then an oil change, it was silent. Go figure.
The design of these engines allows the hydraulic valve lifters to "pump down" when they sit. This happens to most modern cars to some extent, but this engine design is particularly susceptible to it. For whatever reason, once running, the oil has a hard time getting back into the lifters to pump them back up. With all the technical discussions I've had, and read about it, there doesn't seem to be one answer- Volvo offers complete engine sets of lifters for sale, as well as complete replacement cylinder heads. They say that it's usually installed due to a customer's insistence on quieting this noisy engine. Keep in mind, this is the same engine still in use in the brand new S80 cars. So they have a vested interest in making sure this engine has a good reputation. If it truly is miserable and won't go away when the engine warms up, ask to have the dealer listen to it first thing in the morning. Drop it off the night before, and they can hear it cold (when it's worst). MAybe take it to a DIFFERENT dealer, in another town, who has no business interest in it, and see what they think. Wish you luck!
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Rob Bareiss, New London CT ::: "Roterande Fläkt Och Drivremmar!"