I had the exact same problem just before my evaporator went. I was loosing lots of coolant badly, most of it ended up under the front passenger side seat toward the back pillar. The noise occured for about 3 months before I finally had more severe problems (coolant under seat + no a/c) to take it to get checked out.

I didn't have the noise after the evaporator, "hose" (~$140?) and some "o" rings were replaced. The dealership also replaced some of the plastic clips that held up the dashboard because they were broken. However, they did not do a great job of putting the dash back together properly and this may be the case with you since you said evaporator replacement is in the vehicle's history. I had to go back to the dealership again because the inside of the car had oily fingerprints, parts of the dash were sagging and the dash lights went out.

I think the bad "hose" that attaches to the evaporator may be your issue, especially if you have a distinct "popping" or gurgling noise.

Did the records indicate, if anything, in addition to the evaporator were replaced (specific items)?

Funny enough, the "hose" that connected to my evaporator cost more than the evaporator.

I always leave the AC button in the on position as the manual recommends and the blower on AUTO (yes even in the winter). Adjust the temperature with the dials and you don't need to worry with anything else.

I doubt the crackling is anything to worry about, but if you don't hear it when the AC button is on, why not do as Volvo suggests and leave it on? Leaving it on will probably help keep other components in your AC system from going bad.

If you want it completely off, set the blower to zero, that shuts everything off.