Why is my AC making strange noises?
Match the noise to the cause: a hum with no fan is usually a failed capacitor, a loud buzz is often the contactor, a metallic grinding or screech means bearings or a compressor and warrants shutting the system off, and a rhythmic rattle is normally hardware, debris or a duct panel oil-canning.
What is most likely wrong
| # | Cause | How common | What it looks like |
|---|---|---|---|
| 1 | Failed run capacitor — a low hum, no fan | Very common | Condenser hums, fan blade sits still. Do not spin it by hand. |
| 2 | Loose panel, screw or debris — rattle | Very common | Rhythmic, matches fan speed, stops when you touch the panel. |
| 3 | Contactor chatter — a buzz or rapid clicking | Common | Usually at start-up. Pitted contacts or weak control voltage. |
| 4 | Worn blower or fan motor bearings — squeal or grind | Common | Rises and falls with speed. Gets worse, never better. |
| 5 | Duct oil-canning — a boom on start-up | Common | A sheet-metal plenum flexing under pressure. Often a sign of high static pressure. |
| 6 | Compressor failure — metallic grinding or a hard-start clunk | Less common | Shut it off at the thermostat and call. Running it does further damage. |
| 7 | Refrigerant hiss or gurgle | Less common | A hiss can be a leak; a gurgle at shutdown is often normal equalisation. |
What you can safely check yourself
- Identify where it comes from — outdoor unit, indoor air handler, or the ducts. That halves the list immediately.
- Identify when: at start-up, continuously while running, or at shutdown.
- Look for the obvious with the power off at the disconnect: a stick or palm frond in the condenser fan guard, a loose service panel, a screw missing.
- Change the filter and listen again — a starved system is noisier.
- If it is a metallic grinding or a screech, turn the system off at the thermostat and stop. That noise is mechanical damage in progress.
Refrigerant, capacitors and the inside of an electrical panel are not homeowner jobs.
A run capacitor holds a lethal voltage after the disconnect is pulled. Refrigerant handling requires EPA Section 608 certification. And in Florida the work itself is regulated: an air conditioning contractor licensed under Chapter 489 may replace, disconnect and reconnect the power wiring at the dedicated disconnect, and repair or replace disconnects, breakers and fuses on dedicated HVAC circuits — s. 489.105(3). That scope exists precisely because this work is not general-purpose electrical work.
Verify any Florida licence, including ours, at the DBPR licensee search.
Why this happens in this housing stock
Two of these are more common here than the national lists suggest. Capacitor hum, because an electrolytic component sitting outdoors through a nine-month cooling season in Central Florida heat is a consumable — it is the single most frequent part failure on a condenser in this market, at a researched $150–$400 installed. And duct boom, because 57.1% of Casselberry was built between 1970 and 1989, often with a single undersized central return, and a modern variable-speed blower pulling against that return flexes the plenum audibly.
The duct noise is worth taking seriously rather than damping: it is a symptom of high external static pressure, which also costs you capacity, efficiency and dehumidification. Have it measured.
One noise you should not hear in Casselberry: nothing about salt-air corrosion. The city is roughly 40 miles inland, so coastal coil-corrosion copy borrowed from Brevard or Volusia does not apply here.
Seminole County Property Appraiser CAMA extract (Parcels.csv, dated 2026-09-05). Improved residential parcels — single-family, townhome and condominium — in city tax district C1, City of Casselberry. n = 8,563.
What the fix typically costs
| Job | Researched range | What drives it |
|---|---|---|
| Diagnostic visit | $75–$150 | Usually credited. |
| Capacitor replacement | $150–$400 | The hum-with-no-fan classic. |
| Contactor or relay | $100–$450 | The buzz or chatter. |
| Blower or condenser fan motor | $300–$800 | The squeal or grind. ECM motors at the top of the band. |
| Compressor replacement | $1,000–$3,000+ | The metallic grind. Read the compressor page first. |
These are researched market ranges, not our prices and not a quote.
The dollar figures below come from published third-party cost guides — named and linked at the foot of the page — and from the City of Casselberry’s own published permit fee. They are here so you can tell whether a quote you have been handed is in the ordinary range for this work. They are not an offer, and no two houses in Casselberry cost the same to do.
Hearing a grinding noise right now?
Turn it off at the thermostat first, then call. That noise gets more expensive the longer it runs.
Phone: (689) 308-3675 · Monday-Friday, 8:00 AM - 5:00 PM
Where the facts on this page come from
Every figure above is either a primary-source document or a named third-party cost guide. All of it was checked in September 2026. Fee schedules, rebate amounts and market prices change without notice — if you are about to spend money on one of these numbers, open the source and check it.