regardless of what you hear, if the boot is leaking grease/oil, you'll need to get that fixed.

the noise can be coming from any one of several points...

wobbline? check the wheels, tires, and lugs first. while it's parked, grab the tires and shake it back and forth. if you're hearing the clunking noise your bearings might be loose...

for the tires, check to make sure there's no bubbles on the tread. wheels, make sure there's no dents. the lugs, make sure they're on tight.

rack and pinion is a pretty simple mechanism. it shouldn't really clunk. if you don't have any oil/grease, you could hear some noise, but not really clunking.

also check your tie rod ends..