[ShanghaiT]

It seems like the ear cups form a seal and the air pressure causes the membrane to bend and make a crackling sound. It is not electrical at all, and can be done without the headphones even being plugged in. 
 
Have any other Stax users noticed this?

 

[Music Alchemist]

It is somewhat common. May also happen when you move your head.

 

[ShanghaiT]

Thanks for the confirmation!

 

[Music Alchemist]

For more detailed info, check out the 1,500+ posts on the site about it.