Vent, Valve & Volt
Vent: HVAC

AC making a loud or strange noise

A new noise is your AC's way of asking for attention. Let's figure it out. Tell us where the sound is coming from and what it sounds like, and we'll sort the noises that mean shut it off now from the ones you can fix yourself or leave alone.

Also searched as: ac making noise, ac making loud noise, ac making hissing noise, ac making buzzing noise.

Before you start. If you smell gas or rotten eggs, stop here. Leave the house, don't touch switches or use your phone inside, and call 911 or your gas utility once you are outside. If a normally quiet AC suddenly gets loud, or you smell burning plastic or an electrical burning smell at either unit, Trane says to shut the system off and call for repair. If you see smoke or flames, get out, stay out and call 911.
An outdoor air conditioning condenser unit on a pad beside a house with shrubs trimmed clear
Every question and answer on this page, in order

Question 1: Where is the noise coming from?

Walk to each spot while the system runs and listen. Stay a step back from the equipment and keep every panel closed.

Question 2: What does the outdoor unit sound like?

Pick the closest match. If you hear more than one, pick the one highest on the list.

Question 3: What does the indoor unit sound like?

Pick the closest match. If you hear more than one, pick the one highest on the list.

Question 4: What do you hear at the vents?

Pick the closest match. If you hear more than one, pick the one highest on the list.

Question 5: Set the thermostat to Off, then look around the outdoor unit and down through the top grille. What do you see?

Look only, once the fan has stopped. Carrier says a damaged fan blade can be dangerous if the system turns on while you are inspecting it, so leave the thermostat on Off the whole time. Keep your hands outside the grille and leave every panel in place. If you see debris and a damaged blade, pick the damaged blade answer.

Question 6: Do you smell rotten eggs, sulfur or a skunk-like odor, or is the hiss at a gas furnace, a gas pipe or another gas appliance?

Trane says a hiss near a furnace can be a gas leak, and DTE Energy says gas leaks often make a blowing or hissing sound. DTE also says not to rely on your sense of smell alone, because some people can't smell the odor and other scents can mask it. If your home has gas and the hiss seems to come from a gas furnace, a gas pipe or another gas appliance, pick the first answer even if you smell nothing.

Possible results

Need the words for the phone call? Build a what-to-tell-the-technician checklist, or read when to call an hvac pro.