Why a Filter Makes a Clicking or Rattling Noise in a Fan Coil Unit

A clicking or rattling sound from a fan coil unit is often traced back to the filter rather than the fan itself. In small plant spaces, light frames, loose retaining hardware and vibration transfer can turn a minor fitment issue into an annoying noise that is easy to overlook until it becomes constant.

For commercial property managers and contractors, the quickest fix usually comes from checking how the filter is seated, whether the frame has changed shape, and whether the unit is transmitting vibration into the filter rack. The issue is often mechanical, which means it can usually be diagnosed without opening up the whole system.

Start with the filter fit

A filter that is not held firmly in place can move every time the fan starts, speeds up or changes airflow. That movement can create a clicking sound, a light rattle, or a repeated tapping against the frame.

Look for signs that the filter is not seated correctly:

• •The filter is slightly undersized for the opening
• •The retaining clips do not fully engage
• •The filter has shifted after maintenance
• •The access door closes but does not clamp the filter securely
• •The frame seems square, but the filter moves when touched

Even a small amount of play can be enough to generate noise in a compact fan coil installation. If the filter is meant to slide into a channel, check whether debris, bent metal or a damaged edge is stopping it from sitting flush.

If you are unsure of the exact dimensions, how to measure an air filter can help you confirm the size before ordering a replacement.

Loose clips and retaining hardware

Loose clips are one of the most common causes of filter noise in fan coil units. A clip may look secure at rest, but if it has lost tension or is not positioned correctly, it can chatter against the frame once airflow begins.

Pay attention to:

• •Spring clips that no longer grip tightly
• •Bent tabs that allow the filter to vibrate
• •Screws that have loosened in the rack or frame
• •Missing retainers after previous servicing
• •Clips that contact the filter media or border instead of the intended fixing point

On some installations, the clip itself is not the only problem. If the clip presses unevenly, it can distort the filter frame and create a gap elsewhere. That gap allows the filter to move, and the movement often sounds like a light metallic click.

Where access is limited, it can help to compare the current arrangement with the original set-up or a replacement filter supplied to the correct dimensions. If the unit uses a non-standard size, custom air filters may be the cleanest way to restore a snug fit.

Warped frames and damaged edges

A warped frame can cause a filter to sit crookedly in the housing. Once the fan starts, the airflow and vibration can make the uneven edge tap against the casing or guide rails.

Warping can happen for a few reasons:

• •A filter was forced into a tight opening
• •The frame was bent during removal or installation
• •The filter has absorbed moisture and changed shape
• •The rack or access panel is no longer square
• •Repeated vibration has worn the locating points

A damaged cardboard, metal or synthetic frame may not be obvious until you remove the filter and lay it on a flat surface. If the border is bowed, twisted or crushed at one corner, it may never seal or sit properly again. In that case, replacing the filter is usually the simplest way to stop the noise.

It is also worth checking whether the filter is the correct construction for the application. Some filters are designed for stronger framing or deeper pleats than others, and a lightweight product in a high-vibration location may be more prone to movement.

Vibration transfer inside the fan coil unit

In small fan coil installations, the issue is not always the filter itself. Sometimes the filter is making noise because the unit is transferring vibration into the filter rack, access door or holding frame.

This is more likely when:

• •The fan is mounted close to the filter section
• •The rack has thin sheet metal that resonates
• •The access panel is loose or not aligned
• •The filter is touching another hard surface
• •The ductwork or cabinet is amplifying the vibration

A filter that would be quiet in one unit may rattle in another simply because the housing is less rigid. That is why the same product can perform well in one plantroom and be noisy in another.

Practical checks include:

• •Pressing lightly on the filter frame while the unit is running, if it is safe to do so and access is permitted
• •Listening for whether the noise changes when the access door is opened or closed
• •Checking whether the filter is contacting the side channel, grille or door edge
• •Looking for worn foam, missing seals or damaged seating surfaces

If the noise stops when the filter is manually steadied, vibration transfer is likely part of the problem. A better-fitting filter, a firmer retaining system or a small adjustment to the rack may be needed.

Other installation issues that create noise

A clicking or rattling sound is often blamed on the filter when the real cause is the surrounding installation. In fan coil units, the filter may simply be the first part to react to a broader fitment issue.

Common contributing factors include:

• •A loose access panel that shakes at start-up
• •A filter door latch that no longer pulls tight
• •Misaligned guide rails
• •A damaged gasket or missing seal
• •A frame that was installed slightly out of square
• •Air bypass that causes the filter to flutter inside the housing

If the filter media is fluttering, that usually suggests the filter is not being held firmly enough or the airflow is finding a path around the frame. That is worth fixing promptly, because a fluttering filter can wear the frame, damage the clips and make the noise worse over time.

For broader servicing support, filter servicing and commercial filter servicing can help keep the filter section correctly fitted and easier to inspect.

What to do before replacing the filter again

Before ordering another filter, confirm the dimensions, frame style and fixing method. A replacement with the wrong border depth, media pack thickness or clip arrangement can bring the same noise straight back.

A useful troubleshooting approach is to check:

• •The exact opening size, not just the nominal size
• •Whether the existing filter is compressed at the edges
• •Whether the frame shows signs of rubbing or impact
• •Whether the holding clips are still tight and correctly positioned
• •Whether any seals, strips or locating pieces are missing

If the system has a pressure gauge fitted, monitor pressure drop as part of the normal inspection routine. Inspection frequency depends on the system, site conditions and dust load, and you should follow the filter and equipment manufacturer instructions. For ventilation requirements, confirm site needs with the mechanical services engineer or building owner, including any relevant AS 1668.2:2024 considerations.

If the filter itself needs replacement, choosing the right construction matters more than simply matching the size on paper. In some cases, a sturdier frame or a better-suited border type is enough to stop the movement that is causing the noise.

Get the right fit for the unit

A noisy fan coil filter is usually a sign that the filter, frame or retention method needs attention. Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, and we can supply or make filters to suit the space, the fixing method and the application.

If you need help identifying the right replacement, call 07 3800 3448 or request a quote. You can also view our products or contact us for support with the next step.