Also .... you bought a bike with an unspecified rattle somewhere in the front end? ...... Really ...... okayyyyy........
Could be headset or brakes or wheel bearings or stem or bars .... and any of those things failing at speed could cripple or kill you. Good plan.
Headset .... well most likely outcome is that you will ovalize that expensive Ti head tube and the frame will be worthless. Bars suddenly shifting while riding can lead to a nasty crash .... whether they turn when the bike doesn't or the bike doesn't turn when you need it to or if they rotate out from under you .... all good ways to investigate the details of your dental insurance coverage at the very least.
If the brakes are loose, that's fine .... only risk is that they crack the fork or tear loose and gouge a huge hole in the crown, and maybe then wedge in the wheel and send you fling .... particularly exciting on a downhill turn at high speed, and best if there is no guardrail ...... And wheel bearings, who needs 'em? What could happen, the wheel could stop turning at speed, of flop sideways? What could go wrong?
While the fix might be simple, .... it might. or maybe permanent damage has already been done .... maybe the guy has wrecked his head tube and doesn't even know it. This is why I tell folks looking at used bikes to bring a knowledgeable friend along.
Coould be nothing. You might have gotten a really nice ride for a really good price. I'd say most of us here would prefer that to any kind of horror story.
But seriously .... "I just dropped $500 on this bike .... it might have terminal mechanical damage, or maybe not. I am going to ask some friends if it fits me .... what do you guys think?"
If you saw that post, what would you think?