I had both my BB and my crank go bad on my MTB, it was a square taper BB, and what had happened was the nut that holds the crank arm on had loosened and was wholloring out both the crank and rounding the BB. I was told by my LBS that I could tighten the bolt after every ride and it would still end up loosening up (which is what happened for the next 2 rides lol). But I never had a creaking noise like what you are describing. I was getting at you said you keep tightening it. If you are able to tighten it time after time, then that bolt is obviously become loose. I'm not saying go out and buy a new BB and crank right now. I'm just saying that it is something you might wanna consider. What type of connection does you crank have? As I said, mine had the square taper, which is the only type that I know of that can do what I am describing. I don't know how others like ISIS and Octalink (which is what I now have would act if you road them loose.

Edit: also, if the cranks tapers are worn out they could cause wear on the BB so I wouldn't just try them on a new BB for very long because if the crank is also bad you could tear up the BB you are testing them on.