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 the numbers show
Counted from the threads quoted below. How we count
- This comes up 22.5× more often in r/hvacadvice than across the site as a whole.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
- 1run a new wireDid this help you?
- 2use a battery operated thermostatDid this help you?
- 3Removing the soft start kit and running the system bareDid this help you?
- 4Replacing the module with a more expensive modelDid this help you?
- 5Checking the capacitorDid this help you?
- 6Washing out the condenser coilDid this help you?
- 7Turning off power to the unit at the breakerDid this help you?
- 8Spinning the blower wheel by hand to check for grinding or wobblingDid this help you?
- 9Replacing the blower motor and squirrel cage as a DIY project or calling a service technicianDid this help you?
- 10Checking fan behavior with the air conditioning turned off to see if the high speed changes.Did this help you?
- 11Replaced the run capacitorDid this help you?
- 12Dampen mountsDid this help you?
- 13Tweak flats by bending a tiny bitDid this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“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
- 20AC making loud noise again after repair
- 3Loud noise from passenger dashboard when turning air to max
- 6Why does my HVAC condensate pump sound like a jet engine?
- 3Grinding sound when turning off a new HVAC unit
- 5Loud slamming noise when watering system turns on
- 3Loud howling and whistling noise from vents after replacing blower motor