Noisy AC: Buzzing, Screeching, Rattling, Clicking

Check these first

Before you call, narrow it to three things: what sound, where it comes from, and when.

• Where. Stand at the outdoor unit, then at the indoor air handler, then at a supply register. Note which is loudest. For a rooftop package unit, listen from the ground and from the room beneath it. Do not go up.
• When. At startup only? Continuously? At shutdown? Only when the compressor kicks on? Record a ten-second clip on your phone. Technicians appreciate those more than descriptions.
• Thermostat. If the sound is grinding, screeching, or a loud bang, switch the system Off. Running a failing motor or compressor is how a repair becomes a replacement.
• Filter. A whistle or a high-pitched rush at the return grille is often a clogged filter, or one that does not seal in its slot. Replace it and listen again.
• Outdoor debris. From the ground: palm fronds, a stick through the fan guard, a bird's nest, a loose screw buzzing against the cabinet. Remove what you can reach without opening anything.
• Breaker. If the unit hums but nothing spins, turn it Off at the thermostat. One breaker reset at most, and only after the humming has stopped.

What it usually means

Sorted by sound. Location and timing sharpen the guess; a visit confirms it.

Buzzing

Outdoor, continuous: a contactor with pitted contacts, or a capacitor on its way out. Outdoor, humming with the fan still: the fan motor or its capacitor has failed and the compressor is cooking. Turn it off. Indoor, faint: often the transformer, normal at a whisper, a problem when loud. Loose panel screws buzz too, and they are the cheap version.

Screeching or squealing

A high metallic scream from the outdoor unit at startup is a compressor with internal pressure trouble, and it is urgent. A squeal from the indoor blower is usually a motor bearing running dry, which happens sooner in a 155-degree attic than the motor's designers imagined. Belt-driven blowers are rare in newer Phoenix homes, but on an older unit a squeal at start can be a slipping belt.

Rattling

Outdoor: a loose fan blade, a bent guard, debris inside the cabinet, or the unit rocking on a pad that settled after a monsoon season of runoff. Indoor: duct panning against a joist, a loose filter door, or a blower wheel that has thrown a balance clip. Rattles are rarely emergencies. They are also the ones people ignore for two years, right up until the fan blade lets go.

Clicking

One click at startup and one at shutdown is the contactor doing its job. Rapid clicking with no start is the contactor trying and failing: usually a capacitor, a low-voltage issue, or a thermostat fault. Clicking from the indoor unit while running can be a relay chattering on the control board. Steady ticking from the outdoor fan is a blade touching something it should not.

Grinding

Metal on metal. A fan or blower motor whose bearings are gone, or a compressor in its final act. Off, now.

Banging or clanking

A single bang at startup from a rooftop unit can be a compressor slugging liquid refrigerant, which is bad for it, or duct panels flexing under the pressure change, which is harmless. Repeated clanking is a loose or broken part inside a cabinet.

Hissing or bubbling

A hiss at the indoor coil or along the refrigerant lines can be a leak. Bubbling in the lines suggests refrigerant in a state it should not be in. Either way the fix involves a gauge set and a license; see refrigerant leak.

Whistling

Air squeezing through a gap: an undersized return, a filter too restrictive for the blower, or a duct that pulled off its boot in the attic. Annoying rather than dangerous, and a marker of the airflow problems that quietly raise your bill.

What the visit involves

A technician listens with the panels off, checks amp draw and capacitor values, spins motors by hand for bearing play, inspects the contactor, checks refrigerant pressures if a hiss is suspected, and tightens what is loose. Most noises are one-part fixes. The exceptions, compressor and blower motor, are where age matters. A screeching compressor on a fourteen-year-old R-22 system is a replacement conversation, and we would rather have it plainly than sell a part to a system with one summer left. The repair-versus-replace guide shows how we weigh it, and an estimate is how you get a real number instead of a guess. A web page can sort a sound into a category. It cannot put a gauge on your system.

Keeping it quiet

Bearings, capacitors, contactors, and fan blades all get checked in an annual maintenance visit, which is where most of these sounds get caught at the tighten-a-screw stage. If the system has always been loud, it may simply be the equipment: single-stage units slamming on and off are inherently noisier than variable-speed systems that ramp gently. For the deeper reading on what each sound means mechanically, we wrote a field note on AC noises. This page is the triage version.