I have a 2006 XC70. Most of the time, when I turn the engine off, I can hear the turbo spinning down. But sometimes I don't hear it. Shouldn't the turbo always spin when the engine is running and shouldn't it, therefore, always spin down when the engine is shut off?

are you letting the engine cool off after drving it? after spirited driving, ie. fast, the correct procedure is to let it cool down to keep the coking of the oil in the turbo. it might be that that you are hearing, the fact that you aren't cooling it off. you are cooling it off, right?

quick lesson on the turbo, when you driving around say 2-3krpm on the tach, the exhaust gases are spinning the turbo at 150,000rpm plus very very fast! the oil that gets fed to it helps by giving the connecting shaft and bearings something to float on without restrictive friction getting in the way.

the engine pumps this to it. If you switch the engine off without letting it cool then:

the turbo will still be spinning, without oil feed, so it will reduce its life heavily, the oil that is in the turbo will heat up and burn, and leave deposits inside it (also not good) and the heat left over will just generally cause problems especially if your turbo is glowing ahem (guilty).

as greek says let the car settle for about 10-20 seconds before turning the engine off, that way the turbo will spin down and be safe. You can buy turbo timers, which you turn the key off and remove it and it keeps the engine running for a preset amount of time so you dont have to worry.

also, for performance cars they recommend turbo rebuild very 30-60k miles i think but that all depends on what turbo you have :)

but yeah after normal driving wait a few seconds after stopping before turning off, and after spirited driving (italian tune up style) leave it for a minute or two.

Matt