Every HVAC Noise, Ranked From Ignore It to Shut It Off

Last checked: August 6, 2026

Sorting Sounds by Urgency, Not Volume

Loud is not the same as serious. Some of the scariest-sounding HVAC noises — the whole-house shudder of ductwork popping as it warms — are structural small talk, while one of the most dangerous failures announces itself as a barely-audible click-click-click before ignition. Urgency lives in what the sound means, not how it makes your kitchen feel.

It helps to note three things before calling anyone: when the sound happens (start-up, mid-run, shutdown), where it seems to come from (the equipment itself, the ducts, outside), and whether it's new or newly worse. Those three answers, relayed on the phone, routinely turn a two-hour diagnostic visit into a one-hour fix because the tech arrives with the right parts on the truck.

Here's the catalog, from benign to urgent.

Usually Harmless

• Pops and pings from ducts at start-up and shutdown — sheet metal expanding and contracting; annoying, fixable with duct bracing, dangerous never
• A soft whoosh at ignition — a modern furnace lighting normally
• Gurgling from a heat pump in cold weather — refrigerant doing its defrost-cycle business
• A single clunk at shutdown — dampers or the blower settling

Call This Week

• Squealing or screeching — a blower belt slipping or a motor bearing running dry; cheap now, expensive after it seizes
• Rattling from the cabinet — something loose that will eventually find something to break
• Clicking that repeats without ignition — an igniter or flame sensor failing; the furnace that clicks five times tonight stops lighting entirely on the coldest night of February, per Murphy's law of heating
• Outdoor unit buzzing louder than usual — often a capacitor on its way out; a $30 part that takes the compressor with it if the hard starts continue
• Whistling from returns — the system gasping through a clogged filter or undersized return, overheating itself a little every cycle

Shut It Off Now

• A bang or boom at furnace ignition — delayed ignition: gas pooling and lighting all at once. Each boom stresses the heat exchanger, and a cracked exchanger is the carbon-monoxide failure. Off, and call.
• Metal-on-metal grinding — a blower wheel or motor consuming itself; running it converts a repair into a replacement
• A hissing you can hear over the equipment — refrigerant escaping under pressure, or worse near gas lines; either way, not a wait-and-see sound

The Londonderry Wrinkle: Cold Nights Amplify Everything

Noises here have a seasonal rhythm. The first cold snap of October is when marginal bearings start squealing — a summer of humidity followed by heavy load finds every weak part. And heat pumps, increasingly common in Londonderry's newer developments, make a set of normal noises in winter (defrost whooshes, brief growls at reversal) that alarm first-year owners into unnecessary service calls. If your heat pump made the sound while a cloud of vapor rose from the outdoor unit, that was probably a defrost cycle doing its job.

Report a Mystery Noise

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