Just compression tested my Astra 1.6 8v zsr Auto 1999. Working from left to right as you look at the engine - 1st & 2nd cylinders went straight up into the green segment on tester. Went up to around '14' on the dial. Cylinder no.3 went in to the green but was a little slower in getting there and got to around 13.5. Cylinder no.4 was the slowest to get to the green segment at around 12 > 12.5 on the dial.
Whilst the tester was on no.3, there was a 'pop' sound. We both heard it, and it came from the engine and not the tester. The 'pop' sound was like air pressure releasing would sound, not over loud but definately audible!!