The location, selected programme and timing are useful. A whistle is different from a loose shelf rattle, and it should not be investigated by inserting objects into the opening.
What the observation means
Check externally for recently added trim, coverings or nearby items that may obstruct the designed airflow. Do not remove fixed ventilation parts. Note whether the noise appears only during cooldown or throughout cooking.
A useful next step
Keep vents clear as the manual requires and request assessment if the change persists. Stop use if there is overheating or an electrical smell. Sealing the gap to silence it can interfere with cooling and hide the symptom rather than resolve its cause.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.