When looking at the unit. Your HDD is under the middle-right of the touch pad.
Your GPU fan is on the right of the laptop. (CPU on the left respectively)
Confirm that it is your hard drive by listening to it (as it's idle) and then execute something on the drive to turn it on. If the same sound you remember arises, it's simply the HDD working.
If this bothers you, you're more than able to replace the HDD with an SSD. There's tonnes of support here to help you go through it step by step. ;d
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