The Doppler Effect and sound interference
The doppler effect occurs when a moving force is admitting a pitch (frequency) of lets say 1000 hz. When that sound is moving toward you, you will generally hear a higher pitch (a higher frequency). As the sound source moves away from you, you should hear a lower pitch. This happens because as sound source is traveling with the sound waves, therefore, making the sound waves bunched up, and when sound waves are bunched up they admit a higher frequency. The same things occurs on the opposite side, except for the sound source is moving away from the sound waves on the other side, making the sound waves farther apart, thus a lower frequency.
There is an equation to figure out the doppler effect (bottom right picture)