For as long as i've owned my 1999 S4 (8 months) i've noticed that on full lock either turning left or right, the front tyres feel like they 'rumble' as if there is friction as they roll. Both tyres have worn more on the outer edges. Is this a simple tracking problem or maybe something more serious? I'm also planning to upgrade to 18" rs6 replicas & need help in tyre choice. Any help/advice would be appreciated.