The sound you hear is actually being generated by the part of your ear known as the cochlea. It’s a very complicated organ with sensory hairs, internal fluid and nerve receptors, that when damaged (or as it naturally degrades as you get older), can cause it to send incorrect input into your brain. In layman’s terms, because it’s no longer working as well as it used to, it thinks there’s a ringing sound in the area and tells your brain to generate that sound in your head. There are other symptoms of tinnitus, but this is the main one.
Atherosclerosis. With age and buildup of cholesterol and other deposits, major blood vessels close to your middle and inner ear lose some of their elasticity — the ability to flex or expand slightly with each heartbeat. That causes blood flow to become more forceful, making it easier for your ear to detect the beats. You can generally hear this type of tinnitus in both ears.