timed clicking noise on idle

[drew peacock]

just got my camaro out the garage to do something and noticed when idling about every 30 seconds or so something is clicking in and the idle drops a little.
sounds like when you switch on the aircon.that kinda sound..tried it with the aircon on and off but it still does it.
is this normal or am i just being paranoid. i noticed the last time i drove it it does affect it when in drive stationary.
it's something electrical as when the revs drop the interior light goes a little dim.

Edit..just messed with the settings. only seems to do it when the recirculation button is on..no problem as i never use it anyway