Noise from pulley at top of the engine on my Altea Diesel

[MikeD]

Hi All,

Nice to find this site.

My lads got a 2005 Altea Diesel.His first car.

When it starts up the top belt pulley [on the left looking at the engine when stood at the front of the car) moves towards the engine and rubs against the engine block and makes a bad scraping noise then moves away from the block but then moves back every now and again making the scraping noise.
Not sure what it is, has anyone got an idea and also rough potential cost, assuming its a common fault?

Hes not using the car atm.

Thanks in advance.