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Compare the sound with your model’s operating notes
Metal changes size as it warms and cools. Heatilator’s ICON direct-vent manual lists a metallic sound during those temperature changes among its normal operating observations. Look for the corresponding notes in your own manual. That explanation only applies to the kind of sound described and does not clear a new vibration, repeated ignition trouble, or damage for continued use.
Give the technician a three-part description
Was the unit starting, running steadily, or already off? Did the sound come from the fireplace front, from a fan, or from an uncertain area behind the wall? Did it fade or persist? Note any service or remodel work immediately before it began. If you already have a recording, keep it. Do not run the unit again just to reproduce a concerning noise.
Treat an abrupt change separately
Leave the fireplace off for a professional check after a bang at ignition, visible damage, unexplained soot or a new persistent rattle. If you smell gas, leave the building without operating switches and call emergency services and the gas supplier from outside. A routine appointment or online sound comparison is inappropriate during a suspected leak.