Why do I still hear my HDD humming in my PC, if I've since formatted it completely?

[Microsoft Windows]

I installed an SSD the other day and cloned my installation of Windows 10 to it. My hard drive is completely empty now, and it shouldn't be reading or writing anymore. Yet I can hear humming inside my PC, and the HDD activity light still flickers intermittently, if far less than before. Is there another component apart from the HDD that makes noise? I was under the assumption that the drive would only be active if I was deliberately using it to read or write files.

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