Just spitballing,
Pedals are notorious noise machines. On a flat stretch try combinations of 2 and 3 feet on all 4 pedals and see if it has an impact.
You said the sound was high pitched. Does the pitch vary with bike speed or pedal cadence? If the former it is likely in the wheel/hub. If the latter, likely the drive train.
Does the bike have a conventional timing chain or gates belt? I have heard noise from an overly tight timing belt.
Check for a loose bearings/hub. If the wheel is floating on the axle pedaling force could pull the wheel out of alignment.
Check for constant contact of your teams shoes with the crank arms pointing to a mis-adjusted cleat.
Finally, look for anything rubbing the drivetrain. Cables, accessories, chain guards.