Front end wobble
 

I have a 2000 s40. The stearing wheel twitches to the right at very slow speed and developes into a vibration and thumping at high speed. Wife swears she has not hit a curb. Wear seems fine on tires. There is brake noise and a loud humming from the rear at high spead all of which goes away when braking or slightly swerve left. Pads look fairly new but front rotors could use replacing.

Welcome to the forum.

How many miles are on the car? It could be an issue with a worn ball joint or an axle.

Solution found
 

Despite checking the tires several times previously and finding no problems, it was the left front tire. I jacked the front end up, ran the car in gear to see if a rim was bent and found that the left front tire was bulging. Belts had separated.

Nice find. The only other way you would have found that would have been to have them road balanced. Glad that she is all fixed now.