AC makes churning sound

2005 GMC Sierra crew cab with 73K. When I sit idling, the AC makes a "churning" sound, probably from the evaporator. It will continue for a few seconds after shutting off the engine so it's not a compressor problem. The dealer says there's nothing wrong but I've never had a vehicle ac make this kind of noise before. I'm almost out of warranty. I don't remember this noise when the truck was new so I suppose it has happened gradually. Any ideas?

When you shut off the engine with the A/C still on, there is a pressure difference still present between the high and low sides. The hissing/gurgling sounds are just the refrigerant continuing to flow into the evaporator to even out the system pressure.

Personally, I like to shut the A/C off before I shut the engine off, but I haven't heard of any damage done by leaving it on. Most new vehicles keep the A/C clutch disengaged until after the engine is re-started, lessening the load on the starter.