my t9 is making a quiet but annoying clicking sound every few minutes, it's not the system turning on.

Diagnosis

The quiet, rhythmic clicking sound you are hearing from your Honeywell Home T9 is caused by the mechanical engagement of the internal relays. Unlike modern "solid-state" switches, the T9 uses physical electromagnetic switches to complete the 24V AC circuits required to trigger your HVAC system. When the thermostat polls its remote room sensors or performs a "health check" on the circuit continuity, these relays can fire even if the system isn't actively initiating a heating or cooling cycle. This phenomenon is often amplified by the "echo chamber" effect created by the wall cavity behind the thermostat. If the wires are bunched up or the wall plate isn't perfectly flush, the vibration from the relay click resonates through the drywall, making a tiny internal sound audible from several feet away. You can confirm this is a relay issue if the click coincides with the screen refreshing its temperature readings from a remote sensor or if it happens in a precise pattern (e.g., every 5 to 10 minutes).

How to Fix It

Since this is a hardware-level mechanical action, there is no firmware toggle to "mute" the relay. The goal is to decouple the thermostat from the wall to stop the acoustic amplification.

1. Power Down: Turn off the breaker to your HVAC system to avoid shorting the C-wire or R-wire during handling.
2. Remove the Faceplate: Gently pull the T9 display off the wall plate. Inspect the back of the unit for any debris or dust buildup that might be causing uneven seating.
3. Manage the Wire Bundle: Reach into the wall hole and carefully push the excess wiring deeper into the wall cavity. If the wires are pushed tightly against the back of the wall plate, they act as a bridge that carries the vibration from the relay directly into the wall studs.
4. Apply Acoustic Dampening: Cut a small piece of thin, non-conductive adhesive foam or a single layer of electrical tape and apply it to the perimeter of the wall plate (where the faceplate snaps in). This creates a gasket that absorbs the mechanical vibration.
5. Ensure a Flush Mount: Tighten the mounting screws on the wall plate, but do not over-torque them. If the plate is bowed, it will create a gap that acts like a speaker cone, amplifying the click.
6. Reseat and Test: Snap the faceplate back on firmly. Ensure there is a tight, seamless seal between the plastic housing and the wall.

If That Didn't Work

• Check for C-Wire Stability: If the clicking is erratic rather than rhythmic, you may have a "power stealing" issue. If you aren't using a dedicated C-wire (Common wire) and are relying on a power extender kit, the relay may be clicking as it struggles to maintain a steady voltage. Install a 24V C-wire transformer to provide constant power.
• Relocate Remote Sensors: If the clicking happens specifically when a remote sensor updates, try moving the sensor further away from the thermostat. In some interference-heavy environments, the T9 may struggle to maintain a handshake with the sensor, causing the internal logic to poll more frequently.
• Inspect for Voltage Spikes: Use a multimeter to check the voltage between R and C. If you see fluctuations exceeding 28V AC, your transformer may be failing, causing the relays to "chatter" or trip prematurely.
• Update Firmware: While the click is mechanical, some early versions of the T9 software polled sensors more frequently than necessary. Ensure your T9 is connected to Wi-Fi and has automatically updated to the latest version to optimize the polling intervals.

Heads Up

• Avoid Third-Party Shims: Do not use cardboard or thick rubber shims behind the wall plate, as this can put undue pressure on the pin connectors and lead to a "Loss of Power" error.
• Dust Prevention: Every six months, use a can of compressed air to blow out the gaps around the wall plate. Dust buildup in the relay area can occasionally cause overheating, which may change the sound of the click.
• Wiring Integrity: Ensure no bare copper wires are touching the back of the wall plate; a minor arc can mimic the sound of a relay click and eventually damage the T9 motherboard.