If you've checked everything else (seatpost, cranks, pedals...) and it still squeaks under load, check for frame cracks, especially about the bottom bracket shell area, and especially near the welds. All the alu bikes frames I cracked started squealing under load for no reason before failing. If you have a crack, it may be quite hard to see : if it's just starting, it'll be a very thin hairline, so take your time to check.