Noisy ventilation: six causes and what to do

Correctly designed and maintained ventilation is barely audible. When it becomes audible from a bedroom, something has changed — and that something is almost always identifiable.

The good news is that the noise is itself a diagnosis: its character points to the mechanism involved. Here is how to read what you are hearing.

1. A low rumble that crept in gradually

That is the classic profile of a motor bearing reaching the end of its life. The noise builds slowly, over months, which is why people get used to it without noticing — until someone from outside points it out.

A worn bearing gets replaced, and on many models that repair costs a fraction of a new unit. It is the textbook case where a diagnosis prevents a needless replacement.

2. A vibration travelling through the walls

If the noise seems to come from the ceiling or wall rather than the appliance itself, it is a decoupling problem: the unit is in rigid contact with the structure and transmits its vibration into it. The building then acts as a sounding board.

The fix is mechanical and usually inexpensive: anti-vibration mounts, flexible suspension, flexible connectors between the unit and rigid ductwork. On a recent installation it is often an original fitting fault.

3. A high-pitched whistle at one specific valve

Whistling indicates excessive air velocity through too narrow an opening. Two possible causes: a nearly closed valve forcing air through a slot, or duct undersized for the required flow.

The instinct to open the valve wide usually removes the whistle — but it unbalances the system and starves another room of its airflow. The right move is to re-balance the whole system, not to fix one valve in isolation.

4. Noise that rises noticeably in use

Many modern systems switch automatically to high speed when a humidity or CO₂ sensor detects a peak. Noise that rises during a shower and drops twenty minutes later is therefore not a fault: it is the intended behaviour.

That said, if the system stays stuck on high speed permanently, the sensor may be fouled or failing — and there is something to fix, with wasted electricity in the balance.

5. Knocking or irregular thumping

Rhythmic knocking often comes from a backdraught damper flapping, or a detached piece of flexible duct vibrating in the airflow. Irregular thumping, on the other hand, suggests a foreign object in the impeller — which happens more often than people think after building works.

In both cases you have to open up and look. It is not a noise to let settle in: an unbalanced impeller wears the bearing far faster than normal.

6. Noise from outside, through the duct

If you can hear the street, the rain or your neighbour through the extract valve, the ventilation is not making noise: the ductwork is transmitting outside noise. It is common in installations running in a straight line between façade and room.

The remedy is a duct silencer or a change of routing. In an apartment block the same path can carry noise from a neighbouring flat — worth identifying precisely before blaming the neighbours.

What you can check yourself

Those five observations, given over the phone, often let us arrive with the right part already in the van. That is real time saved for you.

1. 1Locate where the noise comes from: the unit itself, a specific valve, or the structure.
2. 2Note whether it is constant or tied to an activity (shower, cooking, night).
3. 3Check the filters: a saturated filter makes the motor work harder and raises noise.
4. 4See whether someone has closed or covered a valve.
5. 5Listen for whether the noise changes when you change speed — if it does not change at all, it is mechanical, not airflow-related.