"Worn shock absorber" noise - but shock absorbers are OK

My 1999 Peugeot 306 has started making an intermittent knocking noise that
sounds like worn shock absorbers (dampers) but these have been checked and
found to be fine. It is an irregular pok-pok pok-pok-pok noise which seems
to correlate with minor bumps in the road; large bumps like speed humps do
not cause it to happen. It happens at low speed (20-30 mph) and seems to be
more common when the engine is cold - ie for the first ten minutes or so of
driving the car in the morning. The noise seems to be coming from somewhere
on the left (nearside - I'm in the UK) of the car, but I can't pin down the
source any more precisely as to whether it's front or back.

When I had the car serviced a few weeks ago, the garage were unable to
reproduce the sound during a road test or to identify from the symptoms I
described what might be causing it. They checked the suspension and steering
mechanism and could find no fault.

Any suggestions?