Diagnosis
That's the water pump cavitating—it's trying to pull water that isn't there. The internal tubing or the connection from the reservoir to the pump has a clog or an air leak.
How to Fix It
Descale the machine immediately. Run a full reservoir of half white vinegar and half water through a full brew cycle, then run 2-3 cycles with clean water to flush. Mineral scale is the most common clog culprit.
If That Didn't Work
Check the silicone seal on the bottom of the water reservoir. If it's cracked, warped, or missing, it's letting air in. You can order a replacement reservoir from Ninja.
Heads Up
This noise puts serious strain on the pump. If descaling doesn't stop it quickly, stop using the machine to avoid burning out the pump motor.