Probably the ABS unit doing a self test. I have an '01 S60 and as you
say, every time you start up and set off, (just after getting into
second gear) there's a funny noise, though on mine it sounds more like
clunk than a click (every trip [1]). Next time you drive it, see if
the vibration you feel is actually coming from the brake pedal rather
than the accelerator.
[1] <http://en.wikipedia.org/wiki/Clunk_Click_Every_Trip>
