Anytime an aircraft exceeds the speed of sound, it creates what's known as a sonic boom. Like a boat pushing aside water as it travels, a supersonic aircraft quickly pushes aside air molecules. This builds up pressure in the molecules and causes shock waves. These shock waves extend behind the craft in two ever-widening cone shapes, one from its front and one from its rear.
The pressure of the molecules releases at the end of each cone, causing a "boom" sound the entire time. When those ends reach the ground, we hear the trademark "boom-boom" of a supersonic jet.