Almost two decades ago, had a squeak, in my case it was with a pedal stroke. One of my square taper crankarms was a bit loose. I was fortunate to fix it in time before any damage done to crankarm.
Could it be an ungreased seatpost or seatpost shim?
Does it squeak if you pedal backwards? Backwards, you would have no tension on the chain or any other drive train components, other than the weight you are applying to the pedals and cranks/bottom bracket.
If you stand next to the bike, for example on the right side of the bike, left hand holding saddle, right hand on handlebar and holding rear brake so bike can't move. Standing on your left food, put your right foot on the pedal and press down on it. Was there a squeak? That is how I discovered one of my strange clicking noises, as when I was standing next to the bike then my ears were better able to tell were the click was coming from. Try the same thing from the other side of the bike.