Tapping Only when idling?
I got a tapping from my brake booster when a non-return in the vacuum line failed so it may be related to another vacuum line.
To Trace source try some tubing, one end to ear and listen while moving the other end around engine bay (don't get it caught in anything)
Or get a stethoscope from a motor supply place (cheap)
Try also
http://espritfactfile.com/ as there are some knowledgable people there.
We have no requirement for smog stuff here so I know nothing about that sorry.