Metal on metal scraping noise
 

Hi guys,

I've got an 04 volvo s40 (first of the newer shape.) The wife took my car to work today and on the way home, approx 10 minutes after setting off a loud metal on metal scraping noise started from the front of the car.
This started when moving and accelerating in 3rd gear.
She stopped on the side of the motorway and couldn't see anything obvious.
She then pulled off under gentle acceleration in 1st through 3rd.
When she changed from 3rd to fourth there was a pop and then the noise stopped. The noise did not come back for the rest of her journey.

Any ideas? Car has done 53k miles