Ringing in the ears can be completely normal or not. Everyone has a certain level of noise in their head, and you can hear it more clearly if you go into a quiet place where you're insulated from the usual sounds around you. Some people hear ringing that's louder than this usual level, which can be from any of the 4 parts of the ear: the outer, the inner, the middle, and the brain.
Unfortunately, as we age, we lose hearing and ringing is a part of this progress towards losing hearing in those sound frequencies. Loud sounds can make this normal process of losing hearing more accelerated, since loud noise damages the nerves associated with hearing.
Sometimes, other conditions cause ringing. Retained fluid (after infection, or sterile fluid with serous otitis), infection, or problems with the eardrum (tympanic membrane) or middle ear bones can cause ringing. less common causes are medications such as aspirin, Meniere's syndrome, aneurysms or acoustic tumors.
In any case, ringing that doesn't go away deserves a doctor's visit. Physical examination with tuning forks, audiometry, and additional radiographic tests can help rule out more serious rare causes.