Yesterday, whiile riding my Saracen MTB, I noticed the cranks creaking whenever I pressed down on the left-hand pedal. I didn't think anything of it at the time, but when I got home, I decided to try a quick test. Pushing the left pedal down fairly hard with the left crank pointing forward made the crank creak once, but then it didn't do it again until I pressed the cranks the other way, I.E left-hand crank pointing backward, hold right-hand crank and press down on left-hand crank. Again, it only creaked once, until I turned the cranks through 180 degrees and pressed again. Each time it creaks, I think the cranks move ever so slightly relative to each other (about half a degree, I reckon). Correct me if I'm wrong, but does that sound like one of the cranks is loose? They're fairly old Shimano Alivio cranks, the bottom bracket is a cartridge one, and I've had the bike several months with no problems from them. Any suggestions as to why this is happening now, and more importantly, how to stop it?
Thanks in advance