Common. Tinnitus - any abnormal sound in the ears is very common. It can be a ring/ a cricket sound/ static, etc. It is many times associated with hearing loss but sometime you can have normal hearing. If it is only on one side... I'd be more concerned - either way it may be best to be seen by an ENT and get an audiogram.
Tinnitus. This is the 64, 000 dollar question. In most cases, we can not give you the answer. We now know that the true origin of the sound is not from the ear but from the highest centers of the brain (auditory cortex). However, it is true that tinnitus is often associated with a hearing loss from age, noise, trauma to the head, certain antibiotics. I strongly urge an ENT visit and get an audiogram.