Is it getting louder over time and independent of braking? Mine got louder and more consistent over time. If your CV boot isn't ripped, I'd start with the bearings. I just had both replaced on my S70. My mechanic told me the volvo front bearings typically start going around 60-70k miles so you're probably due. He recommended replacing them both at the same time since it's likely that the other one will go shortly after the first one.