'02 Sable - ignition pop sound

Hey guys,

I did a search for this but didn't find anything so I thought I'd ask:

'02 Sable V-6.

On cold mornings, I get a "pop" sound when I turn the ignition switch to "on". Almost sounds like it's coming from under the dash board, like someone thumped something against it. It's not affecting the operation of the car but the sound drives me crazy. When I start the car later in the day (when it's warmer) the sound is much less noticeable, sometimes not at all.

Any idea what it can be?

If it means anything, the AC doesn't get cold anymore. I'll be taking it in for a check-up week.

Thanks.