DIAGNOSIS You've got something jamming the blades, likely a piece of frozen fruit or a nut that's too hard. The motor is straining against it, overheating the plastic housing and gearbox.
FIX Stop immediately. Unlock the vessel and inspect the blade assembly. Look for anything wedged underneath it or between the blades. Clear any debris. For future blends, especially with ice or frozen fruit, use the pulse function in short bursts and add more liquid to keep things moving.
IF THAT DIDN'T WORK If the noise happens even with an empty vessel and water, the blade bearing or the drive gear inside the base is likely damaged from past jams. You'll need to contact Ninja support, as the internal parts aren't user-serviceable.
HEADS UP Never run it continuously for more than 45 seconds. The small motor builds up heat fast. Let it cool for a minute between cycles, especially with thick or frozen ingredients.