DIAGNOSIS: That squeal is almost always dry bearings on the idler pulley or the drum support rollers. The idler pulley puts tension on the drive belt, and its bearing is a common wear point.
FIX: Unplug the dryer. Remove the front panel to access the drum area. Locate the idler pulley assembly—it's a small wheel that the belt wraps around, and it's under spring tension. Spin the pulley by hand. If it spins roughly or squeaks, or if the wheel wobbles, it needs replacement. Lubricating it is a temporary band-aid; the sealed bearing inside is failing.
IF THAT DIDN'T WORK: The rear drum bearing (a large glide or a set of rollers) could also be the source if it's lost lubrication. But the idler pulley is the more common and higher-pitched culprit.
HEADS UP: When the idler pulley fails completely, it can seize and burn through the drive belt. If you hear the squeal, address it soon. A broken belt will stop the drum from spinning entirely.