A noise like this can have several causes, and I'll list them in the order of how frequently we see each issue:
1. A flaky solder joint: If a joint is well crimped but not well soldered, it may make noises at random intervals. The best way to check for these is with a cheap pair of headphones plugged into the Crack and a wood chopstick. When you poke the bad joint, you'll get a pretty loud pop. The most common joint that I see causing problems is where the black wires meet on the headphone jack.
2. Dirty tube pints: All 6080s are reasonably old, and we supply old stock 12AU7s. Sometimes the pins can get a bit oxidized, and dirty tube pins can cause some noise. The easiest way to resolve this is to pull and reinsert each tube several times to scrape through the oxide layer.
3. Loose hardware: This is a frequently overlooked aspect of these builds. The hardware needs to be tight, and it needs to be consistent with what's in the manual. The frequency of improperly mounted power transformers was one of the major reasons for the Crack 1.1 revision.
4. A noisy tube: You'll notice this is last. Occasionally, we find a noisy tube or two. Almost always we can get these tubes to quiet down by letting them run for a few days straight, so this is unlikely to apply to your scenario.
-PB
