Might be a slim chance it's tinnitus, but you've would be extremely bothered if you had it and tinnitus sufferers know the discomfort that comes with it. Do you have sinus problems or high blood pressure - both can cause noise in the ears. It's not noticeable until everything is completely quiet.

It could also be from routine Ipod use. I worry about the affects of music from Ipods on young people who have them in their ears, way more than they should. Hearing loss is accumulative - the hair in our ear drums slowly and gradually wear away, from repeated noise.