DIAGNOSIS Inductive humming from the internal power-stealing relay or a transformer under load. It's a physical vibration, not an electronic beep.
FIX Tighten the wire connections at the wall plate.
- Power down your HVAC system at the breaker.
- Remove the display and check every wire on the wall plate. Even a slightly loose wire can vibrate.
- Ensure no stray copper strands are touching adjacent terminals or the wall plate itself.
- Push each wire in firmly until you feel it click into the terminal's grabber.
IF THAT DIDN'T WORK The sound may be coming from your furnace transformer, amplified through the wires. If your system has a Common (C) wire, the buzzing often stops because the Ecobee isn't trying to 'steal' power. Adding a C wire is the permanent fix.
HEADS UP A faint buzz is common, but a loud hum or buzz that changes with system operation can indicate a failing transformer at the furnace. If tightening wires doesn't help, have an HVAC tech check the transformer voltage.