****. iono man. maybe your fork is flexing like a mofo and your wheel is hitting your downtube? ahha.

um. you said it happens when you go over hard bumps. maybe if your tires are a little bit low it's the actual rim touching the pavement. this happens to me when i go over a real bad pot hole or something without anticipating it.

try spinning your wheel freely for a while. do you get any of that noise? if you do, my bet is it's the hub. if you don't, then just keep riding, if you hear it again go to the lbs.