It might be tinnitus. It might also be a bruit in a blood vessel close to your inner ear...I've heard of that happening before in an article I read that caused the same result. A bruit (pronounced: "brew - wee") is turbulent blood flow that sounds like a wooshing or almost like a hurricane going on when listening with a stethoscope, and happens when there is narrowing of the artery. Obviously hearing a sound in your head is not normal, so you should go to a clinic or MD office ...if it is a bruit they should be able to hear it with the bell-end of their stethoscope.
It may be an ear infection. That or it could be you have water in your ears, from taking a shower or swimming in a pool. Just go to the doctor and have them check out your ears. It may seem like it's in your head but I've had swimmers ear (as they call it) and it's a lot like how your describing. Best of luck to you! No matter what it is, get it checked out :)
do you hear it when there is little noise around? generally people with hearing problems will have a ringing in their ears that might block out other noises. My aunt is having hearing problems at the moment, the ringing thing, and she's going to have to get an operation because it's getting worse. you really should get it checked out because it might mean something is wrong with your hearing and the risk of waiting for it to get worse is not a smart decision.
It's possibly you have an ear infection. Go to an ENT (ear, nose, throat) specialist.