Diagnosis
Food particles are trapped in the sealed bearing area under the stainless steel blades. It's a design flaw—they're not meant to be fully disassembled by users, but gunk gets in anyway.
How to Fix It
Fill the jar about halfway with warm water and a drop of dish soap. Screw the blade lid on tightly. Place it on the base and run the blender for 30-60 seconds. The centrifugal force will push water through the bearing seal. Rinse thoroughly. For stubborn bits, use a soft brush (like a clean toothbrush) to scrub around the base of the blades while holding the unit upside down under running water.
If That Didn't Work
For dried-on residue, make a paste of baking soda and water, apply it around the blade base with your finger or brush, let it sit for 10 minutes, then repeat the blending-with-soapy-water trick.
Heads Up
Never submerge the motor base. Never try to pry off the stainless steel blades from the plastic housing; you'll break the seal and ruin it. The blade unit is one sealed piece.