Had and am still having this issue as well... in the Boston winter with very dry inside air. After 3 weeks of moisture build up, there was a crackling sound in Transparency mode or in ANC mode (not in OFF mode). I contacted Apple support via online chat (no wait), and after many check this-and-that steps, they advised me to get a replacement set. They indicated it would be 3 weeks for mail delivery or I could try a local Apple store to see if they had any in stock. I was able to get a Genius Bar appointment for the same afternoon (security-controlled during COVID, strict social distancing), and I received a replacement set that same day.
By this time, with this new replacement pair, I was aware of the condensation issue — and that if left unchecked, it could lead to a deterioration in audio or ANC quality. Armed with that knowledge, I purchased a full-size protective case with a zipper that fully encloses the AirPod Max. Into this new protective case I placed several silicon gel packages, and each night I charge my AirPod Max in this case. Additionally, after a few hours of wear, I’ve gotten in the habit of taking off the ear cushions to check for moisture, and I wipe with a paper towel if/when I find anything. (So far, I’ve found moisture/condensation on the metal edge under the ear cushions every day.) Now, a month later, they’re still going just fine with this (somewhat tedious) daily care.
Hoping someone will develop an improved ear cushion that has some moisture-absorbing properties. I’ve read a rumor that Apple is considering selling a “Sports” version of the ear cushions that could be more absorbent, but I can find no authoritative source on this.
Finally, I thought that perhaps a soft silicone protective cover on the metal exterior ear cups might help equalize the temperature of the metal/ear barrier, but they had no effect on the condensation. Rather, even a tiny bit of the protective cover over one of the microphones messed up with the ANC and the Transparency Mode. Bye-bye, silicone covers!
The Transparency mode is outstanding, and when I switch to my XM4s while the AirPod Max is drying out, I get sad when the speak-to-chat only has 60% of the transparency. Still, both excellent headphones.