MAHLE's workshop magazine puts it simply: "Even a turbocharger in excellent condition makes noises!" A shaft spinning that fast will always make some sound. The useful questions are whether the noise is new, louder or different, and whether it arrives with smoke, rising oil use or a loss of power.
Whistles and hisses: usually a leak
Holset's diagnosis bulletin says that "Often the noise comes from air/gas leakage due to pre-turbine exhaust gas or air/boost leaks", and tells the reader to check all joints first. MAHLE gives typical examples:
- On the charge-air side, a sealing ring reused after a joint has been opened is already deformed. Charge air escapes past it with a whistle or hiss. A stone chip in the charge air cooler does the same.
- On the exhaust side, a reused metal gasket on the manifold flange lets exhaust gas escape.
Garrett's checklist for a noisy turbo adds loose or damaged pipework and support brackets, poor connections, and leaks or cracks in the intercooler. Because escaping boost also costs power, a whistle often comes with the symptoms in our guide to a turbo losing boost.
A whine or siren that rises with the revs
This is the sound most likely to mean wear inside the turbo. Garrett's training notes describe contaminated oil wearing the bearing surfaces until the oil can no longer control the shaft, and say the damage is usually accompanied by a sharp increase in noise, with oil leaking past the seals and smoke from the exhaust. By then, Garrett warns, "the remaining life of the turbo may be extremely short". Metal-to-metal contact follows, then wheels rubbing their housings, imbalance, a bent shaft and often a broken one, and possibly fragments of wheel in the pipework, intercooler and exhaust.
Balance matters too. Garrett balances each shaft and turbine wheel "to ensure reliable, quiet operation at very high speeds", so a rotating assembly out of balance is another source of noise.
Grinding, scraping and rubbing
Anything that hits the compressor wheel, from a loose nut left after earlier damage to debris drawn in through a split intake, damages the blades, and Holset warns that damaged blades upset the rotor balance. MAHLE also describes a less obvious cause: if the air filter is not checked and replaced, intake resistance rises, and in the worst case the rotor shifts along its axis until the compressor wheel brushes the housing.
One sound is normal on some turbos. Garrett notes that on its variable-nozzle turbos it is normal to hear a high-pitched noise from the electric actuator at start-up.
Flutter and surge
The flutter heard when the throttle snaps shut after boosting is part of what Garrett calls surge. On a compressor map, surge is the region of unstable flow, "characterized by mild flutter to wildly fluctuating boost and 'barking' from the compressor". Garrett says surge happens most often in two situations: under load, which it describes as the more damaging of the two, and when the throttle shuts quickly after boosting. Its ported-shroud compressor housings shift the surge line to give the compressor more room before surge sets in.
Checking shaft play
Some play is designed in. Garrett's training notes say the bearing clearances "are often mistaken as being 'excessive play' when examining a new, dry turbocharger", and Holset says they are sometimes misdiagnosed as worn bearings. The better test is whether either wheel has touched its housing. If the clearances are out of specification, Holset points to the lubrication: too little oil, dirt in it, or coolant contamination. Acceptable play varies from one turbo model to another, so the figure for that turbo is the one that counts.
When to stop
Holset is clear: "Never continue to operate an engine with a suspect or noisy turbocharger as this could result in total engine failure."
Rebuild or replace
Once a wheel has touched its housing, the rotating assembly needs new parts and correct balance. Garrett's notes make clear that fragments may already have reached the pipework, charge air cooler and exhaust, and those need clearing before a rebuilt or new turbo goes on. Our turbo rebuilds page covers the rebuild, and turbo diagnostics explains how a noise is traced to its source.