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If it does it for 2 minutes after it is switched off I would guess it is either something that has a time delay or a charged capacitor.
I would pull the battery cable and see if it keeps making noise. If it does I would say it has to be a capacitor charged that is keeping the noise going. If it does keep making the noise after disconnecting the battery I would guess the amp would be the cause but may be something else too.
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