My bike is a 2004 Trek 5200. Full Ultegra, double crankset up front.

I have a moderate "creaking" noise that sounds to me like it's coming from the bottom bracket. Seems to only happen on the upstroke of the left pedal, whether mashing or just riding. I've done the troubleshooting stuff listed on the Park Tools website (check crank arms, chainrings, pedals), but it's still noisy.

Could this be/is it the bottom bracket itself going bad?

I bought the bike used from feeBay, and know nothing of its history...

Please and thanks!
-Matt

Could be a number of things making noise. Sheldon Brown has a good page of reading on creaks and noises - you might want to try and search for that page and read up. I picked up a used Bianchi Limited a while back and it had similar noises but on the right power stroke. Starting with the pedals, then crankset, and then the BB, I overhauled and re-packed everything, making sure to lube or put anti-slip/sieze compound as appropriate on threads and torque sufficiently. I also replaced BB bearings and chain. Now it's whisper quiet and very smooth. It sort of goes to my thinking, which is, a little TLC as soon as I notice noise can often fix the problem and really keep my drive train working well.

I dug through the info on Sheldon's page as well, it seemed to say basically the same stuff as the Park Tool site. Check all of the fasteners and other moving points "down there", lube, inspect, tighten, etc.

And I'm absolutely trying to do "a little TLC now" instead of "major repairs" later.

https://www.bikepedia.com/quickbike/B...5200&Type=bike

Looks like it has a cartridge BB: https://techdocs.shimano.com/media/te...9830608710.pdf

Take the BB out and try this: https://www.bikeforums.net/showthread...ight=cartridge

Or, ride it into the ground and then buy a new one for $20.

While some people do repack external BB bearings, the BB6500 is a very different beast. The top tier BB7700 is designed to be serviced (repacked); the lower level Octalink BBs are not. Additionally, that linked thread discusses cartridge bearings in general, not sealed BB cartridges.

Once the "easy" fixes have been met w/o success, I'd pull the cranks & BB, clean & grease, then reassemble with proper torque. That is, as long as there isn't indication (grinding or movement) of a failing BB.

the only noise i hear is the creaking, and it *seems* to be coming from the left side of the BB. that said, i could be wrong...

no griding or movement that i can detect, either while pedaling or with the bike in a stand.

Your skewers are loose. No kidding; make sure they are super tight. A loose skewer will creak and you'll go crazy trying to fix it with changing pedals, BB, etc.

I saw this problem once on a friend's bike. Worked on the bottom thinking that was where the noise was coming from. With the bike on a roof rack I pushed the fork to make sure it was secure when I recreated the creak.