DIAGNOSIS Air is being forced through a restricted path. The most likely cause is a clogged, missing, or improperly installed post-motor filter.
FIX 1. Turn off and unplug the vacuum. 2. Remove the post-motor filter cover at the very top of the vacuum, behind the handle. 3. Remove the purple cylindrical filter. Inspect it for damage and heavy dust loading. 4. Wash it in cold water only, squeeze out water, and let it air-dry for at least 24 hours. 5. Ensure it clicks fully back into its seal before replacing the cover.
IF THAT DIDN'T WORK A leak in the filter housing seal can cause whistle. Check the orange seal on the filter housing and the corresponding seal on the filter itself for cracks or debris. Ensure the cover is latched securely.
HEADS UP This filter must be completely bone-dry before reinstallation. Even slightly damp, it will restrict airflow and may cause the motor to overheat.