Both sounds are made by the closure of valves. Imagine the valves are doors that when closed make noise. The sound is pretty much like this tum- TA- - tum- TA- - The small silent break between tum-TA is the systole and is made by the closure of atrioventricular valves(because the aortic one has to be open in systole) The big silent break TA- -tum is the Diastole, made by the closure of the Aortic valve. I hope it helped you imagine the process better. Go ahead and search on youtube. Very useful videos there