Record the change from normal
Note when the sound started, whether it happens on every shutdown, whether it is getting louder, and whether there is less airflow or interrupted heating. If the noise occurs during normal operation, a short recording from outside closed equipment can help. Do not cycle the furnace repeatedly to reproduce it.
Trane lists squealing as a possible blower-motor warning. Its broader sound guidance associates rattling with several possible moving or loose components. Neither sound identifies a part without inspection. Trane squealing guidance, Trane rattling guidance.
Ask for the source to be confirmed
Tell the technician that the noise is strongest as the fans slow down. Ask which components are still moving at that point and whether the sound comes from a motor, a wheel, a mounting point, or another attached surface.
If a part is recommended, request the observed condition and why replacement rather than another correction addresses it. Ask whether the affected model has particular service instructions or part-compatibility requirements. A generic sound description is not enough to choose a motor.
Do not spray lubricant through an opening, tighten internal parts, remove covers, or alter fan settings. A quieting trick can leave the actual problem unresolved and can create another fault.
Know when to stop waiting
New scraping, grinding, severe vibration, burning odor, or loss of airflow should move the request ahead of routine care. For a severe new sound, stop heating through the ordinary owner control if it is safe and there is no smoke, suspected fire, or suspected gas leak, then arrange prompt qualified service. Trane recommends shutting down normally quiet equipment when it suddenly becomes loud. Trane changed-noise response.
Your next step is a service request that names both sounds and the exact shutdown timing. Ask for the finished repair to be checked through that same event, so normal heating alone is not mistaken for resolution.