Is a turbo 'whistle' a symptom of turbo failure?

The other day I was driving my 2012 Renault Scenic 1.5 DCi when the Check Anti Pollution System Message came on with a slight loss of power. I managed to get the car to a mechanic. He took the vehicle out on a test ride and everything seemed okay. I was advised to drive as normal and to return if the warning appeared again. On picking the vehicle up, my wife noticed that there was a turbo whistle when changing from second to third and therefore the car was returned back to the mechanic. Have been informed that it's the turbo and have been quoted £700 for this to be sorted. There has been no smoke from the exhaust of any colour and I'm a little dubious as to whether it is the turbo. Any thoughts?
A "turbo whistle" usually indicates a worn and failing turbo bearing, which is pretty much all there is to go wrong with a turbo. They fail because not enough engine oil is getting to the bearing to cool and lubricate it, and the reason for that is usually that the turbo bearing oil feed and oil return pipes are blocked with carbon from switching the engine off when the turbo is too hot. Could be that simply replacing these two pipes will fix it.
Answered by Honest John on

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