How to identify where a creaking/squeaking sound comes from? There is a creaking/squeaking (halfway between each, I would say) sound in my bike. Here is what I know so far:


Happens during pedaling when the left leg goes down
Only when I press sufficiently hard on the left pedal
Not when the pedals are not moving
There is no discernible issue with the the moving parts involved as far as I can see: on a stand, rotating either pedal moves the wheel freely, with no extra friction that I can feel, the wheel on its own turns freely as well
The pedal and crank arm are made of some sort of plastic
I checked Strange long squeal sound when I pedal hard? I can&#39;t figure out where it&#39;s coming from and I don't think it is the same issue as:


The squeaking is not very high pitched
The pedals don't wiggle
The sound happens exclusively on one side



Based on that, I suspect something is wrong either in the bottom bracket, or in the left crank arm or pedal.

How do I test which part is the issue, and how catastrophically can this fail?

Edit

As it turned out, the crank was coming loose. It started unscrewing itself, and eventually fell off. It was hard to understand at first, because I didn't put enough force in trying to see if it was wiggling. I now suppose that it was (just a little), but only when putting my weight on it.
As a bike mechanic on a college campus, I got this question a lot. From my experience, it usually is either the pedal or the bottom bracket. Considering the noise is happening when you are putting torque on your pedal, I would go with it being a bottom bracket problem. Check for play by pulling on the crank back and forth to see if there is some wiggle room, but even if there isn't, it could be that it's just dry in there and you're feeling the ball bearings more than usual. 

If you've never taken apart a bottom bracket, I would consider getting some help first. If you mis-align the threads screwing the cup back in, you could seriously damage your bike. I'm not sure what type of bottom bracket you have, but if they are loose ball bearings, I would give them a re-greasing, and then make sure to tighten it up so there's no play while also having it loose enough to turn with the greatest ease possible. Even just doing that could get rid of some noises. 

You could also take apart your pedal on the left side and check the ball bearings in there too, and give them a re-grease. 

If this is a bottom bracket problem like I predict, and you let it go, your ball bearings will eventually turn into misshapen balls and that will dent the cups in which they roll, which will just totally suck to ride because it will be hard to pedal and annoying to listen to the persistent noise. I doubt it would ever make you crash though. 

This is a personal picture I took of a bottom bracket with the same complaint as yours, and these are their ball bearings and cage. I'm not sure how he rode it for so long like that actually.


Hope this helps!