Nine times out of ten it is a worn serpentine belt. Tensioners are also know to make a rattling noise congruently. Changing your belt and/or tensioner should solve your problem.
Check the belts - they're probably loose or worn.
check your wheel bearings
The most common cause is slipping belt
Most common culprit is a slipping belt.
A worn belt will cause a squealing noise. The squealing noise is caused when the belt slips. Tightening the belt will temporarily fix the problem.
rear differential got lube? rear wheel bearings?
Bad belt. Worn tensioner. Bad compressor.
Its almost sounds like a dolphin noise.
The belt has worn on the edges and creates friction which causes squealing. Also, sometimes when a belt is cold it will squeal.
It could be the transmission, it could be some parts under the vehicle that rub only when travelling forward.
A Geo Metro is a front wheel drive vehicle. When a squealing noise occurs when the wheel is turning, it can be an indication of damage to the CV joints.
Check for a slipping drive belt between encine crank pulley and AC compressor pulley.