Cracking noises from the housing and a faint 'rasp' – is the unit dirty or is the plastic expanding?
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Berceni, Alunișului street, 1984 block, 12,000 BTU Samsung, 4 years old. Two noises: dry cracks at start and stop, and a faint continuous rasp while it runs. The cracks I understand (expansion), but the rasp? It appeared together with a slight smell. Related?
What is really happening
The polystyrene housing and copper pipes change dimension by a few tenths of a millimetre between 25 °C and 8 °C; the plastic slips on its clips and cracks – at start (cooling down) and at stop (warming up). That is materials physics, not hygiene. The rasp is different: air passing through an evaporator with fins partly clogged by felt and through a roller with blades thickened by biofilm becomes turbulent, and turbulence sounds like a rough, continuous rustle proportional to speed. Evaporator studies show fouling raises pressure drop and cuts airflow by 5–7% – exactly the flow change that produces the sound. That it appeared with the smell is no coincidence: the same biofilm layer produces the odour's volatile compounds. Two symptoms, one cause.
- The casing and fasteners.
- Vibration transmitted to brackets or the wall.
- The fan and possible deposits or abnormal contact.
- A typical evaporator fouls enough to double its pressure drop in about 7.5 years, reducing airflow by 5–7% and raising fan power by 1–10%. [Lawrence Berkeley National Laboratory / , 2002]
- The 'dirty sock' or musty smell from an air conditioner comes from volatile organic compounds produced by microbial biofilm on the evaporator and blower. [Current Microbiology, 2000]
- Mould odour is treated by WHO and epidemiological studies as an indicator of active microbial growth, even when nothing is visible. [PLOS ONE, 2012]
- The blower (cross-flow fan) is the most fungus-contaminated component of a split unit, more than the filter or the front of the evaporator. [Indoor Air, 2023]
What we do on site
- We listen and confirm: the cracks remain after sanitising, the rasp does not.
- Collection cover; cleaning solution on evaporator, roller, pan; pressure wash in place.
- Sanitising solution.
- Outdoor unit, washed in place.
- Test at medium speed, with you – clean air through a clean evaporator sounds like a soft 'ssss', not a rasp. See current rates on the Prices page.
What you can do yourself
- The cracks: if they bother you at night, low speed and 'Sleep' – temperature swings are smoother.
- Filters every two weeks, 'Fan' before shut-off.
- Unit off at the breaker, remove the filters and listen with your ear at the grille to how the sound changes – a simple 'dirty/clean' test.
What NOT to do
- Do not call a repair service for the cracks – not a fault.
- Do not stuff foam or 'anti-vibration' tape into the housing – you block drainage.
- Do not ignore the rasp if it comes with smell.
Prevention
Annual sanitising and the unit stays at its day-one sound. The expansion cracks you will have for the unit's whole life – they are normal.
When to call us
0764 012 943, Monday–Saturday 08:00–20:00. Berceni.
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Read also
Sources (7)
- www.osti.gov/servlets/purl/805122
- link.springer.com/article/10.1007/s002840010120
- link.springer.com/article/10.1007/s002849900435
- pmc.ncbi.nlm.nih.gov/articles/PMC6968652/
- journals.plos.org/plosone/article?id=10.1371/journal.pone.0047526
- www.ncbi.nlm.nih.gov/books/NBK143940/
- onlinelibrary.wiley.com/doi/10.1155/2023/8984619
The information on this page is educational and based on cited sources; it does not replace medical advice. We do not do repairs, installation or refrigerant circuit work.