DIAGNOSIS That's the motor straining against a locked-up drive system. The most likely culprit is something jamming the blade assembly or the jar being misaligned on the base.
FIX First, unplug the blender. Grab the jar and try to turn the blade assembly by hand—it should spin freely. If it doesn't, you've got something wedged in there. Unscrew the blade assembly from the bottom of the jar (it unscrews counter-clockwise) and clear any debris. If the blade spins freely, the issue is alignment. Look for the arrow on the jar and the corresponding mark on the base. They need to line up perfectly. When you set the jar down, give it a firm press and a slight twist until you feel it click into place.
IF THAT DIDN'T WORK If the blade spins freely and the jar is aligned, the drive coupling on the base might be stripped. Inspect the black plastic square peg that sticks up from the motor base. If the corners are rounded off or it looks chewed up, that's your problem and the base needs service.
HEADS UP Never run the motor for more than a few seconds when it's humming like that. You'll overheat and potentially burn out the motor. Always stop, unplug, and investigate the jam.