Why is my air conditioning noisier when heating is on?

You switch to heating.
The sound changes.
It feels louder than before.

This worries many homeowners. In most cases, the noise is normal winter behaviour rather than a fault. Heating mode places different demands on the system, and those demands create different sounds.

Heating mode creates different sounds

Heating and cooling do not sound the same.

In heating mode, the system
Moves more refrigerant
Works against colder outdoor air
Adjusts fan and compressor behaviour

These changes alter sound levels. A noise difference does not automatically mean something is wrong.

Outdoor units work harder in winter

Cold air makes heat extraction harder.

The outdoor unit must work longer and sometimes louder to draw heat from colder air. This often produces deeper or more noticeable sounds than cooling mode.

This behaviour increases during colder mornings and evenings.

Defrost cycles add temporary noise

In winter, the outdoor unit periodically defrosts.

During defrost
Fans may change speed
The system may pause briefly
Steam may appear outdoors

These actions create short bursts of unfamiliar noise. They usually last a few minutes and stop once defrost finishes.

Fan speed increases perceived noise

Fan speed affects sound directly.

Higher fan speed
Moves more air
Creates more audible airflow

If heating feels weak, many users increase fan speed. This improves heat spread but also increases noise.

Medium fan speed usually balances comfort and sound.

Airflow direction affects how noise travels

Airflow direction matters.

When warm air blows straight down or across hard surfaces, sound reflects more easily. This makes the system feel louder even if output stays the same.

Adjusting airflow direction can reduce perceived noise without changing settings.

Expansion and contraction sounds are normal

Heating warms internal components.

Metal parts expand as temperatures rise.
Plastic panels shift slightly.

This can create clicking or ticking sounds during start up or shut down. These sounds usually stop once temperatures stabilise.

Why noise seems worse at night

Background noise drops at night.

When the house is quiet, system sounds stand out more. Outdoor temperatures are also lower, which increases heating demand and system activity.

The system behaves the same.
Your environment changes.

Common user actions that increase noise

These behaviours often amplify sound.

High fan speed used continuously
Auto mode changing behaviour
Frequent on off switching
Very high temperature settings

Each one causes the system to work harder or less steadily.

Noises that are usually normal

These sounds are common during heating.

Steady airflow noise
Low humming from the outdoor unit
Brief whooshing during start up
Short clicking sounds

They should be consistent and not increase over time.

Noises that should not be ignored

Some sounds need attention.

Grinding or scraping
Loud rattling
High pitched squealing
Electrical buzzing that persists

These are not normal and may indicate loose components or electrical issues.

When to seek professional support

Contact a specialist if
Noise suddenly becomes much louder
New sounds appear and persist
The system vibrates excessively
Error codes appear

Before calling, note when the noise occurs, which mode is active, fan speed, outdoor temperature, and whether the sound is constant or intermittent.

This helps diagnosis.

Related guidance

Further residential air conditioning guidance is available at
https://www.climateworks.co.uk/residential-air-conditioning

References

UK Government guidance on heat pump operation
https://www.gov.uk/guidance/heat-pumps-how-they-work

Building Regulations Approved Document L
https://www.gov.uk/government/publications/conservation-of-fuel-and-power-approved-document-l

ClimateWorks residential operational observations from installations across Hampshire, Berkshire, Surrey, and Oxfordshire between 2022 and 2025

Author

Dr Julian Carter
Technical and Compliance Director
ClimateWorks

Dr Julian Carter has over 20 years of experience in building services engineering, air conditioning system design, and regulatory compliance. He advises on residential and commercial projects across the UK, covering system selection, installation standards, commissioning, and real world performance.

As Technical and Compliance Director at ClimateWorks, he oversees technical governance, installer training, fault diagnosis, and customer education. His work focuses on reducing user related issues, improving system reliability, and ensuring air conditioning systems perform consistently throughout their service life.

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