Describe what happens before the heat
From observations already made safely, note whether the sound begins before warm airflow, whether the furnace eventually heats, and whether a fault appears. Include the outdoor conditions and whether the noise occurs only during the coldest weather. Use an existing recording from outside closed equipment if available.
Cold timing does not establish that the sound is normal. The technician needs the exact model and the actual sequence, including what is active before heating begins. Do not open the furnace, bypass a pressure switch, change gas settings, or repeat resets to force another start.
For example, Lennox's SLP99DFVK owner manual calls for professional inspection when unfamiliar sounds recur and notes that burner operation can produce unusual sounds. This is an example of an equipment-specific warning, not a remote diagnosis of your furnace. Lennox changed-sound instruction.
Make the waiting plan specific
Tell the provider the indoor temperature, how quickly it is falling, and whether anyone needs extra help staying warm. Ask when service is actually available. If the home cannot remain suitably warm, arrange a warm place to stay and ask an appropriate professional about protecting the home from freezing damage.
If using a portable electric heater, follow its instructions, keep it at least three feet from combustibles, and plug it directly into a wall outlet. EPA heater precautions.
A portable heater may serve a room but does not establish that the whole house or concealed pipes are protected. Turn it off when leaving the room or sleeping. Use another warm arrangement if the available temporary heat cannot be used safely.
Your next step is a service request centered on the cold-weather startup noise, with the household's heating needs stated. Ask for the actual finding, operating recommendation, and correction before treating an online sound match as the answer.