Betty is so right about the high doses of aspirin. But not the dosages used to make your blood more slippery or for regular headaches. That would be very unusual.
There are other medications that can cause it, and of course Les's old friend from the '70s, classic tinnitis, which is damage to sensor hairs in the cochlea from exessive sound pressure (noise).
As you are probably aware, there is no actual sound involved, even though you "hear" it. Think of it like a broken key on an electronic organ, that always stays on, whether pressed or not.
One very loud noise or repeated exposure to loud noises can cause it. Or sometimes, one of the hairs just breaks from age, and starts sending its assigned tone permanently into your brain.
There are some medications that may help alleviate it. I've actually seen eardrops for it, but I can't imagine that they could work, given the actual location where the "sound" is generated.
There are numerous tinnitis websites around. They may help with figuring out your cause.
Best wishes,