I had this same noise on my 2010 WRX when depressing or releasing the clutch when it had 20k miles.
Brought it to the dealership. They tried greasing and oiling up everything that could possibly cause the noise, but it didn't work.
Dealer inspected the firewall for integrity of spot welds and all were of sufficient quality to both my satisfaction and the dealer's satisfaction.
So they finally replaced under warranty a portion of the pedal assembly and the clutch master cylinder and it fixed the noise. At 47k miles now and the noise has not come back.
By the way, the clutch itself felt fine when the noise was happening, and I noticed no ill effects other than an assault on my eardrums.