Diagnosis
The filter was reassembled while damp or was not seated correctly. A whistling noise is caused by air leaking past the filter seal.
How to Fix It
One: Power off the vacuum. Two: Remove the filter by twisting it counter-clockwise. Three: Verify it is completely, bone-dry. This requires a full 24 hours of air drying. Four: Reinsert the filter and twist clockwise until it is firmly locked. You should not see any gaps.
If That Didn't Work
The seal (a purple or gray gasket) on the filter or inside the filter housing may be dislodged. Inspect both rings and ensure they are seated in their grooves properly.
Heads Up
Never use the vacuum with a wet filter. Beyond poor performance, moisture can be drawn into the motor, causing a critical failure not covered by warranty.