Mechanical noise- Precision 3551

Hi

I keep hearing a noise from my Precision 3551. Some observations below:

- It doesn't seem to be coming from an audio output. Present when the laptop is mute. 

- Sounds to be somehow mechanical. Like an struggling HDD (although my laptop has SSD only)

- It is not consistent. Gets lauder when I move the cursor or scroll (with mouse or touchpad)

- When I unplug, it gets noticeably more quite

Any thoughts on what it could be, or a solution, would be really appreciated. Let me know if I can provide more details to help.

Thanks