Speculation, but this seems to have helped my hiss.
The screws that hold on the back of the earcup seem to have developed rust, likely as a result of sweat and skin oils.
I wouldn't be surprised if this also produced a small conductance between the two pins of the pressure feedback microphone, this microphone is located on the speaker grill and directed towards the speaker. Carefully lifting the protective tape and cleaning the connection with rubbing alcohol seems to have done the trick. I also disassembled the right earcup to air-dust out any debris that might have fallen into it or the NC microphone grill.
The clips that hold in the earpad are located in the top, top-left, top-right, bottom-left, bottom-right and bottom. You can somewhat see where the clips clip in by carefully pulling away the pleather from the edge of the can. Remove these clips carefully by using a plastic spudger. It shouldn't require much force, just some wiggling around, you will hear a soft pop.
If you want to dust the inside of the cup, or just the NC microphone grill, take out only the 4 silver screws that hold the back of the cup on. Carefully lift the cup up as the touchpad ribbon cable is still attached, grab the ribbon cable between your fingers, try to cover the entire width of the ribbon cable as you carefully pull it up uniformly to remove it from the connector, a small wiggle may be required, never use excess force.
When reassembling, turn the screws until they start becoming stiff to turn in. If possible carefully, give it a tiny bit of an extra turn to ensure properly sealing the cup (there is a rubber grommet for splash protection). Never use excessive force when inserting screws, you will break the plastic inserts if you do.
Take note of which earpad goes with which cup, they should only fit one way onto each cup, compare the plastic stubs on the earpad with the recesses in the cups.
Be careful not to damage the drivers by dropping/inserting anything past the plastic grill.
Note that any damage done by unauthorized repair will not be covered by warranty. There seem to be no warranty void stickers, but Sony might still attempt to void your warranty if you service your own headphones, beware.
