Why is there a howling noise from my turbo when I accelerate?
There are a few possibilities such as a momentary lack of oil slightly wearing the bearings and causing imbalance or impact damage to one of the wheels, however the most common cause for a howling type noise is head-droop...
It is a problem most of us males have experienced at some point in our life due to excessive alcohol, however it differs in cause for the turbocharger. Head-droop in a turbocharger is when the head of the turbine wheel droops and is no longer perpendicular to the turbine shaft to which it was welded via a process called friction welding. (Mouse-Over Image)
In nearly all cases this occurs due to repeated hot shut downs and extreme temperature, fatigue or combination of both. The turbine wheel head is made from a casting and is relatively heavy in comparison to the shaft. If it is repeatedly shut down when extremely hot it may drop slightly and it only need drop a few thousands of an inch to result in a very loud howling noise which is caused by the resulting imbalance. A turbocharger with head-droop will eventually fail.
If having your turbocharger repaired, remember to advise the turbocharger specialist of any noises as it is quite difficult to measure head-droop given there are few if any flat surfaces on a turbine wheel when trying to micrometer such a small amount of movement. If I find a turbine wheel is near or over 0.10g of imbalance I suspect head-droop as the most likely possible cause and contact the customer to enquire whether the turbocharger was noisy or not.
It is possible to balance a turbine wheel with head droop however the wheel will obviously still be offset and it is unlikely the noise will disappear hence its very wise to replace it. I always advise a customer of this circumstance and highly recommend replacing the turbine wheel as it will void any warranty if the customer insists on taking a chance with a suspect turbine wheel.
It is far cheaper to replace a turbine wheel rather than chance one with head-droop only to find yourself removing the turbocharger again to replace it anyway!