These are not as loud as first crack, more difficult to hear, but they happen a lot more frequently and in greater numbers than first cracks. Second crack takes place over a longer period of time as well. They start slow and then build to a point where there may be one or two a second for a bit and then decrease in frequency. In some blends active second can happen two or more times during a roast. I will remind you that particularly with second crack, the time, rate, and duration can vary dramatically based on the variables I mentioned earlier.
Second crack is very important.
Many home roasters use this as THE indicator when roasting. For most people's tastes, roasting to somewhere around the beginning or at active second crack makes a good espresso roast. It may take the first two or three roasts, but eventually you will be able to easily hear the cracks. Get together with someone who can point them out for you. If you have done a lot of motorcycling, shooting, operating heavy equipment or power tools, or other activity with a high noise level then you may have a hearing deficiency.