Dettson furnace occasionally emits a whistling noise from the return or supply grilles.

Safety first: Turn off power before changing blower wiring.

• 1. Replace filter: Use a less restrictive filter if the current one is high-MERV and causing noise.
• 2. Open registers: Fully open supply registers to reduce velocity and whistling.
• 3. Check grilles: Replace very small or decorative grilles with higher free-area models if possible.
• 4. Adjust blower speed: On PSC motors, move to a lower speed tap for heating if allowed by Dettson specifications. On ECM, have a technician adjust airflow.