you wont hear it spool up, and if u are used to the HP from the turbo then u should immediately feel the loss in power
You would know right away mainly because your car wouldn't run at all. If a turbo is seized or not pushing boost then your car wouldn't be able to make it up hills or accelerate.
to be politicly corect a turbo charger dosnt just stop working, as a turbo would carry on working even if the bearings and oil seals were shot main factors oil seal thust bearings if the thust bearing fail you will get a lot of noise and the circle motion will cause vibration and proberly chip blades on the housing, or if the oil seals fail then you would emit light blue smoke on idle and on deacceliration, but yes you would find there would be no boost
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