Originally Posted by
Sageous
^^ Yes, you very likely could have been asleep, or close to it, when you noticed that you could not hear your AC. You also might simply have allowed the sound of the AC to exit your current realm of perception, and its sound just wandered back on its own... but we won't go there; let's assume, for the sake of WILD, that the former happened and you were indeed falling asleep.
What should you do, should this happen again (and it likely will!)? Basically, nothing.
That's right, do nothing: Don't be surprised, or excited, or even terribly curious. Instead, just sort of notice that the AC noise went away for a time, which could mean you were asleep, or close to it, while still retaining your waking-life self-awareness. Acknowledge that your state might have transitioned from wake to sleep, and that there might be a LD waiting just over the horizon. The good thing about this is that, even if the AC's sound just eluded your attention for a bit, you still mght be putting yourself in a good mental position to LD anyway.
Also, keep in mind that no matter what happens during your night's sleep, there is never a need to force yourself awake. If you understand fully that nothing can harm you while you are safely in your bed, and that odd events like that AC sound disappearing are simply stuff that happens during the going-to-sleep process and truly have no real power unto themselves, then you might be more able to stay calm throughout the process, and maybe enjoy a WILD along the way.
tl;dr: You might well have been asleep, so if it happens again, stay calm and maintain your current state: there is no need, ever, to be surprised, or force yourself awake; especially when you might be on the verge of a LD.