01 / IMPACT
A short knock
A knock says little on its own. Notice whether it follows a faucet or an appliance valve shutting off, or whether it happens when no water is running at all.
HOMEFRONT's note: A bang right after a valve shuts off is often called water hammer: a pressure surge when moving water is stopped suddenly. Whether that is what you are hearing is a question for a plumber.
Don't slam valves or open walls to chase it. A knock that keeps coming back, grows louder or shows up with a leak is one for a plumber.
02 / CONTINUOUS
A steady hum
A steady tone often comes from equipment doing its normal job. What counts as normal for a particular unit is written in its manual, not in a sample.
From the source: ENERGY STAR's heat pump water heater guidance says these units have a fan and a compressor that can make a modest amount of noise while heating water, that sound levels vary among products, and that the unit should not sit next to bedrooms or living areas.
A hum that suddenly changes is a question for whoever services that equipment. A burning smell is different: the Massachusetts Department of Fire Services lists it among the signs to call the fire department immediately.
ENERGY STAR: Heat Pump Water Heater Design Considerations ↗03 / IRREGULAR
An uneven rattle
Does it start when heating or cooling switches on? Is it new, constant or tied to one setting? Those answers help more than a guess at which part is loose.
From the source: ENERGY STAR lists a noisy heating or cooling system among the signs that it may be time to call a professional contractor, and names two possible causes: an undersized duct system or a problem with the indoor coil of the cooling equipment.
A rattle doesn't point to one part, and it isn't a cue to open electrical or mechanical equipment yourself. Start with the manual, then a qualified technician.
ENERGY STAR: When is it time to replace? ↗04 / TEXT ONLY
A buzzing switch or outlet
This card has no sample, on purpose. A buzzing, crackling or sizzling switch or outlet isn't something to compare against a recording: however it sounds, the next step is the same.
From the source: The Massachusetts Department of Fire Services lists a sizzling or buzzing sound, arcs or sparks, and a vague smell of something burning as warning signs to call the fire department immediately.
Our advice: stop using that switch or outlet, don't open it up or try to make it happen again, and follow that fire service guidance: call the fire department. Have a qualified electrician check it afterward.
Massachusetts Department of Fire Services: Electrical Fire Safety ↗