I ride Speedplay X pedals and when the clear springs are getting worn where they contact the pedal axle they can squeak. It took a lot of miles, miss tried paths (pulley wheels and chains were cleaned and lubed a number of times) and frustration each time I thought I had found the cause. It was placing a dab of grease on the pedal axle where the cleat spring can touch that proved where the noise was coming from.

I've heard/seen seat posts creak, BB and crank bolts do the same. But these types of causes generally have a short and "tight" noise and not a longer "drawn out" one like a squeal. But one person's squeak is another's creak. Andy.