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Clanking noise seems to be from bottom bracket region
Hi all. I'm new to repairing bicycles and hoped some one might be able to assist me with an issue I'm having. I recently bought a 1984 Centurion LeMans RS that needed some minor repairs.

Lately it has started making a clanking noise, something like the sound of striking metal when it's under tension, at certain times while pedaling. The noise only occurs when I am actively pushing on the pedals. When I start pedaling from a full stop, the sound is louder. Sometimes the sound occurs when I am not pedaling hard, but it is not nearly as loud and happens rhythmically which each full turn of the pedals. Any idea what is going on here or how to fix it?

Thanks so much in advance!
First a picture of your bike would be very helpful. Then maybe a description of the area where the sound is coming from. Like your wheel, cranks, etc. Easy way to find out is to turn the bike upside down the pedal it. Listen to where the clunk is coming from.
Good maintenance to your Bike, can make it like the wheels are, true and smooth!
Given that the noise is once per pedal revolution, there are a finite number of choices:

Most likely it is your bottom bracket (BB). Your BB needs to be rebuilt - taken apart, cleaned, checked for wear, new ball bearings and re-greased (I use boat trailer wheel bearing grease). Or you can replace your BB with a sealed unit - about $25- plus installation, it is not serviceable, but will last many thousands of miles.

Note: special tools are required for the BB.

Next most likely candidate is the pedals; some are easy to rebuild, some are just trash. If rebuildable, same procedure as the BB.

3rd; front derailleur rubbing on the chain guard, or the chain ring rubbing on the chain guard

Last is a cracked bike frame; not likely on a classic steel frame like yours.
I had a noise like that on my bike once. Checked everything , nothing amiss. Noise still there, finally I noticed that my pump mounted on the lower bar slipped and was being hit by the pedal. Duh.......
Turn the bike upside down like Bill says.put blocks under handlebars if levers are in the way and turn pedals slowly look carefully to see if they are hitting anything, also make sure the chain and the rear cluster are clear. Could be BB like Nigel says,

PS send Bill some pixs....:-)))
Never Give Up!!!
Thank you to all for responding! Your comments lead me googling in the right direction and it turned out the problem was really beyond my skill level. The bottom bracket was loose and a ball bearing had actually cracked, which the guy at the shop said was unusual. In any event, it seems to be fixed but it still makes the very occasional clank from the same region.

You might check to see if under heavy load the crank arm is hitting the frame on the chain stay or hitting the front derailleur. This area can flex enough that this wouldn't happen unless you're putting heavy pressure on the pedals. Also should check to see if pedals are tight to crank arms (best to remove, regrease threads, and reinstall). Could also be bad pedal bearings.
OK, so first of all, thank you to every one for your helpful insights. I will be trying them out over the next few days. It's definitely not the pedal or the crank arm striking anything though. These noises are internal.

I have made a video to help. http://www.youtube.com/watch?v=8SCS0hEvYA0

I couldn't get the rear derailleur to do the thing that it usually does... which is act like its stuck between gears no matter where it's at, especially in lower gears.

Toward the end of a 15 mile ride I did today, it started making a rhythmic clank that was coming from somewhere in the rear wheel region. I checked for obstructions, removed and replaced the wheel and pumped up the tire all the way but it had no effect. At any speed over 5 mph it now makes this new sound.

Thanks again for all your help. This bike is driving me nuts. lol
Hmmm. Clunking could be bad bearing in rear hub. You made it sound like it happened in time with crank rotation, but sounds like it happens even when coasting, so not BB.

The rattle from the front? Could be it the brake lever? Try jiggling stuff or holding stuff while the noise is happening to see if you can spot it. (Ball bearing inside the bars? A favorite mechanic trick to drive coworkers crazy.)

I think you have friction shifting so don't know what would cause it to not sit in gear well. Maybe the chain is way worn. Try measure for "chain stretch" and check to see if derailleur is badly bent.

Looks like a decent older bike, definitely worth investing a little time/work. But probably neglected if it has multiple bad bearing sets.

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