Any noise you hear that is not a real sound that others can hear is tinnitus. The actual experience for people with the condition varies, for some it's a hiss, for some it's a tone, for me it's a really loud, multi-tonal, warbling sound between 11khz and 15khz. If anyone has tinnitus and wants to know what frequency it is that you your brain is perceiving just go online and find a tone generator and start increasing the frequency until the sound from the speakers suddenly disappears. That's the frequency of your tinnitus.

e: btw tinnitus is considered hearing damage.

For me, it's a very distinct ringing. Like in the movies when there's some explosion/shock scene and there's a very persistent ding that happens after the explosion or whatever - it's what I hear constantly.