I don't find AI servo the most usefull focusing mode, so may not be the best indicator depending on camera body.
I've taken lots of pictures where i was focusing on an animals head and gotten it's tail in focus with AI servo... it's just too sow to keep up.
The 7D has a much more acurate system, but i've never taken the time to bother testing it, just because i don't like shooting with it activated.
p.s. I've never heard a lens make a clicking type noise like that. the Non-usm lenses are lounder, but they don't sound like a machine gun with no bullets in it.