its called tinnitus, its a side effect of the APAP in the vicodin. its probably not a good thing, it could mean you are damaging your hearing (i say it could mean because im not sure). both oxy and hydrocodone are supposed to cause hearing loss in the long term (according to wiki anyway) so making a switch probably wouldnt make much of a difference unless you could get oxy without apap. heres a little exert from wikipedia about what you might have heard if what you were experiencing was tinnitus
"Tinnitus can be perceived in one or both ears or in the head. It is usually described as a ringing noise, but in some patients it takes the form of a high pitched whining, buzzing, hissing, screaming, humming, tinging or whistling sound, or as ticking, clicking, roaring, "crickets" or "tree frogs" or "locusts," tunes, songs, beeping, or even a pure steady tone like heard in a hearing test.[5] It has also been described as a "wooshing" sound, as of wind or waves.[6]"