Heating and cooling # is it normal for HVAC to make noise? Each sound points to a different cause: a squeal is usually a belt or motor, a rattle a loose panel, a hum an electrical component. Two signals are not diagnosis but emergency — a gas smell and a carbon monoxide alarm. In those two cases leave the building and call from outside before anything else. ## Why does this matter? What each sound and smell indicates — including the two that mean leave the building. ## What does each sound and smell indicate? Two signals are not diagnosis but emergency, and they come first. A **gas smell** and a **carbon monoxide alarm**: leave the building, call 911 or your gas utility from outside, and do not operate switches. Carbon monoxide has no smell, so the alarm is the only warning you get. The rest can be read with reasonable confidence: - **High squeal** on start-up: blower belt or motor bearings. - **Rattle**: a loose panel, a dropped screw, or debris in the outdoor unit. - **Hum** with nothing starting: an electrical component, often a capacitor. - **Bang** on shutdown: compressor damping, or duct expansion. - **Musty smell** on start-up: moisture on the coil or in the ducts. - **Burning smell** on the first heating day: dust on the heat exchanger. If it persists, shut it down. The dividing line is clean: identifying a noise is safe, opening the cabinet to chase it is not. ## What should you know about refrigerant? Refrigerant circulates in a sealed loop. It is not consumed the way fuel or oil is, so a system that is low on charge **has a leak**. Adding refrigerant without finding and repairing that leak treats the symptom, vents the new charge over the following weeks, and costs you the same money again next season. It is also regulated. Only Section 608 certified technicians may purchase refrigerants intended for use with stationary refrigeration and air-conditioning equipment. The restriction covers ozone-depleting substances and non-ozone-depleting substitutes — including HFCs — in cylinders, cans or drums. Wholesalers must record the purchaser's name, the date of sale and the quantity. There is one exception that confuses people: small cans of refrigerant for *vehicle* air conditioning are still sold without certification. That exception is for cars and does not extend to the equipment in your house. For new equipment, the 700 GWP limit in force since January 1, 2025 is why current systems ship with R-454B (GWP 465) or R-32 (GWP 675) rather than R-410A (GWP 2088). R-410A inventory built before 2025 can still be legally installed — EPA removed the installation deadline on May 21, 2026. [The full timeline](https://hvacbuddy.shop/refrigerants/). ## Why do the ducts matter as much as the equipment? Equipment capacity is rated at a specific airflow. If the ducts cannot move that air, the equipment cannot deliver what its label promises, and the homeowner concludes they were oversold. The number that reveals it is **static pressure**: the resistance the blower works against, measured in inches of water column. It is to a duct system roughly what blood pressure is to a person. A contractor who measures it before quoting a replacement is doing the job; one who does not is assuming your ducts are fine. Duct design has its own ACCA standard, **Manual D**, and it is the step most often skipped on a replacement because the ducts already exist and reusing them costs nothing up front. [What Manual D determines](https://hvacbuddy.shop/ductwork/manual-d/) · [ducts and airflow](https://hvacbuddy.shop/ductwork/). ## What should a bid show you? Three bids rarely quote the same job, so comparing headline price compares three different things. The one that was done properly shows its work, and you can check it line by line: - Model numbers for the outdoor unit, indoor coil, and air handler or furnace. - The **AHRI reference number** for that exact combination. A rating applies to a matched system, not to an outdoor unit on its own. - The SEER2 and, on a heat pump, the HSPF2 of that combination. - The Manual J result the sizing came from. - The measured static pressure and any duct work included. - The permit, and who pulls it. - Labour warranty, separate from parts, and whether registration is required. [The full red-flags checklist](https://hvacbuddy.shop/hiring/). ## What actually drives the cost? We publish no national average price, and the reason matters: an average across every house, region and system type is not a number you can act on. A replacement that includes duct modification, an electrical upgrade and a permit is a different job from a straight swap, and both read as "system replacement" on a one-page proposal. What actually drives your number: - **Equipment capacity and tier**, which should come from the Manual J rather than the old nameplate. - **The ducts** — whether they can move the air the new equipment needs, and what it costs if they cannot. - **Electrical work**, if a circuit or panel change is required. - **Permit and inspection**, usually required. - **Refrigerant** — current equipment uses R-454B or R-32; remaining pre-2025 R-410A stock may price differently. [What a bid should itemise](https://hvacbuddy.shop/costs/) · [operating cost calculator](https://hvacbuddy.shop/calculators/). ## How can you verify any of this yourself? Everything regulatory on this site traces to a primary source and carries the date it was read: DOE for efficiency standards, EPA for refrigerants and Section 608, the IRS for tax credits, and ACCA for load calculation. The sources for this page are listed at the end with the date each was consulted. If you find a different figure elsewhere, check two things before deciding which to believe: whether the page says which generation its efficiency metric belongs to, and whether it carries a visible review date. Most of the outdated content in this category fails both tests. ## Related questions - [why is my HVAC so loud?](https://hvacbuddy.shop/questions/noises-and-smells/why-is-my-hvac-so-loud/) - [can HVAC cause carbon monoxide?](https://hvacbuddy.shop/questions/noises-and-smells/can-hvac-cause-carbon-monoxide/) - [can electric HVAC cause carbon monoxide?](https://hvacbuddy.shop/questions/noises-and-smells/can-electric-hvac-cause-carbon-monoxide/) - [can HVAC leak carbon monoxide?](https://hvacbuddy.shop/questions/noises-and-smells/can-hvac-leak-carbon-monoxide/) - [why is my HVAC making noise?](https://hvacbuddy.shop/questions/noises-and-smells/why-is-my-hvac-making-noise/) - [why does my HVAC smell musty?](https://hvacbuddy.shop/questions/noises-and-smells/why-does-my-hvac-smell-musty/) - [How correct equipment size is decided](https://hvacbuddy.shop/sizing/manual-j/) - [What SEER2, EER2 and HSPF2 mean](https://hvacbuddy.shop/efficiency/) - [Refrigerants and the phasedown](https://hvacbuddy.shop/refrigerants/) - [Incentives: what expired and what remains](https://hvacbuddy.shop/incentives/) Sources (2) - U.S. Department of Energy — [Central Air Conditioners — Conservation Standards guidance (Final, October 5, 2022)](https://www1.eere.energy.gov/buildings/appliance_standards/pdfs/2023_CAC_Standards_FAQ_10-5-2022_Final.pdf) (accessed 2026-08-17) - Air Conditioning Contractors of America — [Manual J Residential Load Calculation](https://www.acca.org/standards/technical-manuals/manual-j) (accessed 2026-08-17) By [HVACBuddy Editorial Team](https://hvacbuddy.shop/authors/editorial-team/). Last reviewed: 2026-08-18. HVACBuddy publishes independent information. We do not install or service equipment, we do not dispatch contractors, and we do not sell leads. Nothing here is engineering, tax, or legal advice.