Yukon clacking noise when A/C is on, and car just underway

This is the weirdest. I have a 2003 Yukon XL, 58K miles, good shape. The A/C blows cold air just fine and no real problems with it. BUT, after cranking the car, backing out the driveway, put in drive, and the first 100-200 yards while accelerating, I have a weird noise under the hood that sounds like something is caught in the radiator fan.

It makes the sound for 5 secs or so, and stops from then on while you’re driving. Remember the playing cards you’d put in your bicycle spokes with a clothespin, to make it sound like a motorcycle - that’s the sound.

It is more likely to make the noise when the care is not warmed up.

If I back out, and drive w/ the A/C OFF, it will not make the sound. I thought it might be the inside blower, but that wouldn’t make sense with the cold/warm car. I thought it might be the radiator fan, but I didn’t see anything (and why would it quit anyway). I checked the wheel well - nothing. The serpentine belt looks fine. It sounds generally like the noise is coming from the passenger side, and near the firewall.

Any ideas would be appreciated? I have a road trip soon and want to prevent a disaster on the road.