Those clicks - actually peak to -22dB below full scale, are always the same shape, 8 samples long, but with ringing in the tail. I'd guess at a static discharge within a circuit capacitor - or in a connected mic. Have you tried swapping the mics over to the opposite channel, or connecting a different input device (or none)? And retesting in a different location, also being sure you don't have a mobile phone too near?