The sound can change as permitted food is processed, so variation alone does not establish a fault. What matters is whether the unit completes the normal task without straining, stopping, unusual impacts or poor drainage.
What the observation tells you
Avoid using motor sound as a reason to increase the feed rate. Let the model's operating limits guide the load, and stop adding material if the sound becomes persistently abnormal. A stationary hum is different from the changing sound of normal grinding.
A useful next step
Record a new pattern together with the food and water conditions. If it continues during correct use, have the appliance assessed rather than testing it with progressively harder scraps.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.