Citroen C5 Aircross Resonator

You hear it before you see it — a raspier, louder note, a rattle over bumps and often a mid-range drone that makes long trips wearing. Underneath, look for perforation along the welded seams and pinholes where condensate has eaten through from inside. The drone is the diagnostic one, because that is precisely the frequency the resonator existed to cancel.

It depends on whether your car uses a bolt-in section or a welded one, and on whether a used or aftermarket part is available for the model. Rather than quote a generic figure that would be wrong for most cars, ask through our form with your model and engine code and we will price the actual section plus fitting. On many cars a fabricated replacement is the better buy.

Midway along the system, between the catalytic converter and the rear silencer, usually under the floor toward the back of the cabin. On the C3, Xsara and C3 Aircross it is the smaller canister sitting ahead of the larger rear box — if you are looking under the car and see two boxes, the front one is the resonator.

In the short term yes, because a failed resonator affects noise rather than how the car drives. Two things argue against leaving it. A resonator that has rusted through is usually the first section of a system that is uniformly tired, so the rest is close behind. And a corroded section can break away and drop onto the road, which is both a hazard and a roadworthy failure.

The silencer is the main box and reduces overall volume across the whole range of frequencies. The resonator is smaller and tuned to cancel one specific frequency — the booming drone that appears at a particular engine speed and would otherwise make the cabin unpleasant. Citroens generally have both, working in series, and each does a job the other cannot.