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Summarizing, you had a failing blower that finally failed completely. The usual cause is bearing failure - and the noise you describe is typical of this problem. This blower is crucial to the processor cooling and, without it working, the processor is overheating and (eventually) throttling performance to protect itself.
First of all, this protection is fragile. High temperatures can still damage the processor and can also damage other components around the processor. I thus recommend that you run it as little as possible and that you get this blower replaced as soon as possible.
Hope this helps,