I fixed a crackling distortion noise problem on one earcup on my Denon AH-MM400

[Blín D'ñero]

The problem had started shortly after i had replaced the ear cushions.

I fixed it, after i did more obvious things first.

1. Took off the cushion, and unscrewed the speakerplate. Nothing immediately obvious to be seen, but under magnifying glass it turned out that the coating of one electrical wire was a tiny little damaged. I wrapped a short piece of insulation tape over it.
But that, in the end, did not fix it.
2. Took off the cushion, and under the magnifying lamp i checked for the tyniest damage i could find. I noticed nothing. Unscrewed the speakerplate and inspected, but again nothing wrong. Assembled it again. Did not fix it.
3. Used contact spray (Deoxit D5) on the banana plug and the adaptor. Did not fix it.
4. When those steps did not help, i took off the cushion, and again under the magnifying lamp i took a VERY close look and noticed a black spot on the front of the speaker conus. It was only now that i realised that i already before had seen that the black grate covering the speaker had a little paint-wear-off, where the black paint was gone. At first i had thought nothing serious of it. But this time, i decided that the tiny black spot on the speaker conus could very well be a loose speck of paint, ruining the clean sound of the speaker.
I used an air-sprayer (or dust sprayer) and it quickly removed the spec! I cleaned the speaker surface nicely out. And this fixed the problem!

Hold the spray can upright during spraying, while holding the ear-cup upside down.