DIAGNOSIS That whistle is almost always an air leak. The vacuum is pulling air through a tiny, unintended gap, and that's creating the noise. It's not typically a motor failure sound.
FIX Go connection by connection. Unplug the vacuum first. Start where the hose plugs into the canister—make sure it's fully seated and clicked in. Check the bag compartment lid—is it latched securely and evenly? Is the bag installed correctly and the paper seal intact? Then check the hose-to-handle and handle-to-powerhead connections. A slight twist or reseating at each joint can eliminate the leak.
IF THAT DIDN'T WORK Inspect the hose itself for cracks, especially near the ends where it bends. Also, look at the rubber gaskets and seals on the bag compartment door and hose connector. If one is deformed or missing, it'll whistle.
HEADS UP Running the vacuum with a persistent air leak makes it work harder and less efficiently. It's worth tracking down to keep your filters clean and performance high.