Diagnosis
That's the sound of the base's suction motor struggling, usually because something is blocking the airflow path between the robot's dustbin and the base's dustbag.
How to Fix It
First, remove the robot from the base and take out its dustbin. Empty it and check the small rubber flap valve at the bottom of the bin—make sure it opens and closes freely. Next, open the Clean Base door and remove the dustbag. Shake it gently to redistribute debris, check for clogs at the bag's opening, and re-seat it firmly. Look inside the empty bag chamber for any obvious obstructions.
If That Didn't Work
The issue might be in the robot's exhaust channel. With the dustbin removed, look into the cavity in the robot where the bin sits. Use a flashlight to check the small metal suction inlet hole for a clump of hair or debris and clear it with a long, thin tool.
Heads Up
Never use third-party or non-genuine dustbags in the Clean Base. The fit and filter material are specific, and off-brand bags can cause clogs and make the motor overwork, leading to that exact noise.