Why is my AC making a loud noise after changing thermostats?
Replacing a thermostat can trigger a loud thumping or banging noise from the AC compressor right as it turns on. This unexpected compressor noise creates disruptive disturbances, particularly when the outdoor unit is positioned right outside a bedroom wall.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
- 1run a new wire
- 2use a battery operated thermostat
- 3Removing the soft start kit and running the system bare
- 4Replacing the module with a more expensive model
- 5Checking the capacitor
- 6Washing out the condenser coil
- 7Turning off power to the unit at the breaker
- 8Spinning the blower wheel by hand to check for grinding or wobbling
- 9Replacing the blower motor and squirrel cage as a DIY project or calling a service technician
- 10Checking fan behavior with the air conditioning turned off to see if the high speed changes.
- 11Replaced the run capacitor
- 12Dampen mounts
- 13Tweak flats by bending a tiny bit
In their words
Unedited, most upvoted first, each linked to the thread it came from.
“Second I turned on the car after the install the fans kick on with way more power than my last cooling fans before they broke.”source ↗
“Just curious if the fans working this hard is normal or if it’ll cause problems.”source ↗
“The ac unit outside has started making an increasingly loud noise the last few weeks. It makes this noise when starting up, then goes away after 10 minutes or so. I read this could be an electrical issue and possibly A fire hazard?”source ↗
“It seems loud and harsh”source ↗
“Makes this noise every time it starts up.”source ↗
“for some reason my compressor is making a thumping sound while running.”source ↗
“The Condenser is right behind my bedroom wall and I hear it banging randomly as it runs.”source ↗
“The contactor doesn't chatter on or off its just the compressor thumping...thump......thump..thump”source ↗
“Sounds like a weed wacker/chainsaw engine in the bathroom and bedroom upstairs when A/C turns on. It's coming from the furnace, not the external A/C as far as I can tell.”source ↗
“That's the blower motor, probably the bearings going. Water damage from that chimney leak doesn't help. It rusts everything out.”source ↗
“I changed out my Nest thermostat to a Ecobee Enhanced thermostat and my AC Condensor started making this load noise when it turns on.”source ↗
Where this came up
People with this problem also raised
- 4AC making loud noise again after repair
- 3Why does my HVAC condensate pump sound like a jet engine?
- 2Why does my car make a clicking noise and not start?
- 17Why does my car run terribly and stutter after a long drive?
- 3Lights flickering when the AC turns on
- 3Is it safe to drive with a noisy AC compressor pulley?