most if not all racks make a clunk when sitting on the spot, things to check though that can cause this vibration at high speeds to do with the rack would be inner and outer track ends, First of all if its bad you should be able to do a test with out taking the wheel of or jacking up the car, Grab the wheel at the 3 and 9 positions, push the wheel in and out and listen and feel for any clunks or knocks, If your not or are happy with this test then jack up the car take the wheel of, have a look at the track rod ends look for torn rubber on the track rod balljoint, last check is to grab the trackrod end itself and pull hard back and forth and listen and feel for clunks, this will check inner and outer ends.
In saying all the above, it could be wheel balancing that is needed, thats the most common cause of vibration at speed 50-70mph
Have a look around other suspension parts while your there, bottom balljoint and wishbone bushes, also a bad tyre and or a bent rim would cause a wobble but generally youd feel something at lower speeds too.
So have a look at the track rod ends to put your mind at ease, if your happy, take it to a wheel fitter place, ask them to balance your 2 front wheels, shouldnt cost anymore that 10-20 quid, have them look at your tyres and alloys when the wheel is of to.
