I've noticed that, when I first start my car, there is an intermittent popping or clicking sound coming from up near the engine (sometimes it almost sounds like a very quick burst of air escaping a high pressure valve - sort of a quick PFFFT). It happens maybe every 6 or 7 seconds when the car is accelerating or moving. It will not happen when the car is idle. About 5 or 6 minutes into my drive, the sound will stop.
This issue started this Spring (4 or 5 months ago) and has been getting progressively worse over time.
I've noticed that the issue only occurs when I have the A/C on. If the A/C is off, I never hear the noise. There does not seem to be anything wrong with the A/C, however. It's still cold, the fan runs fine, etc.
Any ideas what this could be? I'm mostly wondering if it's an annoyance or something that I should be worried about. If it's just an annoyance, I can live with some noise - I wouldn't be driving a Kia Rio if I wanted luxurious transportation. However, if it could be an indication of a broader problem with my car, I obviously want to fix it before it gets worse and causes a potentially more costly repair.
(I've asked about this issue before, though I had significantly less information at the time, and I thought it was engine-related.)