I can't say how many times that has happened to me, it's probably the chain. All you have to do is grease it, first turn the bike upside down and start turning the pedals and while you are turning the pedals so the chain is moving start spraying the chain with WD-40 and when you think you have sprayed it enough you are done, the squeaking should stop. Now, for best results, you can try this, once you have finished spraying the chain with the WD-40, you can also spray the chain with some Silicon Spray, this will just help lubricate the chain even more. But the Silicon Spray is only optional. These tips have a 99% chance of working, they always have worked for me. Hope it will work for you.