Single leg squats, knee making noise

Hello! New member here. Background is 20 years of martial arts, and now I've been training gst for a while. 

So I can do 5 reps of SLS on both legs, but my right knee makes strange popping sounds at the bottom position. So far no pain. Any ideas? is it a flexibility issue? 

Strengthwise I can squat with 120kg at bodyweight of 75kg, with good form, so I probably am strong enough.. but this is squatting to just below knee height, not ass-to-grass. 

Ideas how to fix this? Or what should I do? Conditiom the knee with an easier exercise?

Hi Sami,

Welcome! The single leg squat progressions from the foundation series gradually and safely prepare the joints for squatting. I strongly recommend checking out Foundation One. 

A bit of crunching and popping can be OK if there's no pain. In some cases it is just bubbles forming in the synovial fluid due to the vacuum that forms under loaded knee flexion.

Apparently problem is the extreme ROM in pistol squats. Tried shrimp squats today, and knee was fine. Some years ago I cycled for about 120km in one day, and this particular knee was very sore after that, and I have some pain while cycling. Basically I pretty much quit cycling after that..