A loose access panel or register can create a harmless rattle, while a squeal may come from a blower belt or motor bearing. A delayed boom at ignition is not a normal furnace sound and deserves prompt attention because it can indicate ignition or burner trouble. Scraping, grinding, or a new vibration can point to blower or wheel damage. Turn the system off if you smell gas, hear a harsh electrical buzz, or see smoke. Do not lubricate a motor unless the equipment instructions specifically allow it. Record when the sound occurs, whether the heat output changes, and how old the furnace is. Those details help separate a repair from replacement.
What to ask next
The right next step depends on the system, the urgency, and the building. Keep the model number, symptoms, age, and any previous service notes nearby when you request an estimate. A professional should explain the diagnosis or sizing basis in plain language.
When to bring in a professional
Use the callback form to describe the issue, or review HVAC services to find the closest starting point. For urgent loss of heat, see what to do in a cold snap.