Seventh Circle Audio

so i've got a full rack of sca pres (2 n72s, 3 a12s, and 3 t15s). everything's been working wonderfully for me for the last year and a half or so since i finished my last build, but over the last month or two one of my t15s has started making loud popping noises any time i change the gain. i've rerun the preliminary tests in the assembly instructions and everything's come up as it should. i also sucked all the solder off and reseated the gain knob just in case i was shorting something there, but there doesn't seem to be any change. interestingly, the pop gets quieter as the gain gets louder. i've posted a clip here:

http://www.lansingrecording.com/external/t15gain.wav

it starts with the preamp on full...each pop is me stepping it down a notch. any suggestions on what to try next? i'm a decent solderer but an electronics idiot and the pre seems to be working well otherwise, but i've been avoiding using it since this whole thing started and finally have a little down time this week to try to address the problem.

Measure the voltage between U1, pins 1 and 8. It should be very close to 0V. If it isn't, check your soldering around U2, R22, R23, R6, R7, C8, and C9. De-fluxing the board may help. If it is close to 0V, the circuit is working properly and whatever pops you're hearing are normal.