This kind of noise from the bottom bracket area can spoil a perfectly good ride. Like all creaking sounds, investigate it straight away – bicycles rarely complain unless something is loose, worn or about to snap. If your bicycle has the courtesy to give you warning creaks, it’s worth your time to pay heed.
Fat-tubed aluminium bikes amplify the smallest sound. Anything with any tube fatter than you can get your hand around is, basically, a soundbox, and it will do its best to ensure you hear everything. The end result is that you start lusting after a narrow-tubed Italian steel racing bike, then suddenly you’re visiting your mother for Christmas in full Lycra, duck shoes and aero helmets. Tighten your cranks instead. Try these decreaking measures, and then test to see if the creaking has gone away. If nothing works, note that frames transmit noises strangely, so creaks can sound as if they come from somewhere else. Common causes include handlebar and stem bolts, and rear hubs.
Step 1: Tighten both crank bolts clockwise. They both need to be tight – you will need a long (at least 200mm) Allen key, not just a multitool. The 8mm Allen key on multitools is for emergencies only.
Step 2: If that doesn’t work, remove both crank bolts, grease the threads and under the heads, and refit firmly.
Step 3: Tighten both pedals. Remember that the left-hand pedal has a reverse thread – see the Pedals section for more details.
Step 4: If that still doesn’t work, remove both pedals, grease the threads and refit firmly. This sounds far-fetched, but it does the trick more often than you’d imagine. Dirt or grit on the pedal threads will also cause creaking, so clean the threads on the pedal and inside the crank.
Step 5: Take hold of each pedal and twist it. The pedal should not move on its own axle. If it does, it could well be the source of the creak and needs stripping and servicing. Spray a little light oil, like GT85, on the cleat release mechanism. Don’t use chain oil – it’s too sticky and will pick up dirt.
Step 6: Remove the crank and chainset, loosen the left-hand bottom bracket cup, tighten the right-hand cup firmly (remember it has a reverse thread so tightens anticlockwise), then tighten the left-hand cup (normal thread, tightens clockwise). Refit crank and chainset, and tighten bolts firmly.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Bottom brackets – regular checking for play and wear”