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Parts for Frigidaire GLET1031CS0 Washer Dryer Combo - AppliancePartsPros.com
Often it is the drum rear support bearing assembly, see the Upper Cabinet section.
It may be worn or may just need cleaning and re-lubing with hi-temp grease.
You could check it by removing the rear cover and putting a drop or two of oil on it. If the squeak changes or stops then you know this is the problem. The oil will just be a temporary fix. Be careful that the ball bearing (item 5) does not take off on you when you do this.
Also could be the front drum support, the idler wheel or the motor itself but the rear support is the most common.
If you do pull it apart check all parts for wear so you can replace them if necessary. You may as well give it a bit of an overhaul while you are working on it.
This would also be a good time to clean/check your vent sysstem.