I had an issue recently on my S60 - when the car was first started from cold, there'd be a rhythmic thumping noise from under the bonnet.
Lifting the bonnet, the auxilary belt tensioner was jumping backwards and forwards as slack came into the belt and then was taken up.
Further investigation showed that the Alternator Pulley had started to fail - modern alternator's no longer use a solid flywheel, instead using a clutch-based pulley called an Overrun Alternator Pulley (or OAP). The pulley manufacturer (INA) recommend that it be changed whenever the aux belt is changed.
My aux belt had been changed at the result of an emergency cambelt change (following a water pump collapse). Unfortunately, in VIDA, Volvo list the pulley as a separate service item, rather than listing it as being part of an aux belt change, so the garage missed it.
Because of the time it took to track down and obtain a suitable pulley, the belt was pretty badly chewed by the time I was able to swap the pulley, so I opted to change that as well.
This documentation details the steps required to change the alternator belt and pulley on a Volvo S60 (D5 engine). All images should be clickable.