DIAGNOSIS: A high-pitched whine or whistle is almost always an airflow issue. Something is forcing air through a tiny gap or a clogged filter, creating that noise. It's the vacuum equivalent of whistling.
FIX: Start with the easiest fix. Remove the dust bin and check the rubber gasket seal around the top of the bin and where it mates with the vacuum body. Make sure it's clean and seated properly—a misaligned bin creates a leak. Next, take out the main filter and the fine dust filter. Give them a firm tap to dislodge dust. If they look really dirty, wash them (with cold water only) and let them dry fully. A clogged filter is the most common cause of whining.
IF THAT DIDN'T WORK: Inspect the main air pathway for a partial clog. Look down the tube from the bin area, through the wand, and into the floor nozzle. Even a small object like a cereal piece stuck partway can create a whistle.
HEADS UP: If the whine turns into a grinding or screeching sound, stop using it immediately. That points to a failing motor or a foreign object stuck in the impeller fan, and running it will cause serious damage.