You may send your Precision 7520 to a local laptop-repair shop, ask the shopkeeper to diagnose your laptop fans, if necessary, buy a pair of "precision 7520 fan" from ebay and replace the fans.

By the way, my Precision 7520 cannot detect external monitor, could you please try connecting your Precision 7520 to an external monitor, if it's working then tell me your system information by running  `systeminfo` in Command Prompt. (e.g. Windows 10.0.19044.1415, Intel HD Graphics Driver 27.20.100.8681, BIOS 1.22.0)

I've had this issue with my 7520 when it was new in Jan 2019, and Dell replaced the fan assembly under warranty. It has worked fine ever since.

https://www.youtube.com/watch?v=VSzbcYU00os

If you intend to buy a replacement fan assembly yourself, be sure you buy the same part number as you have now, as there are different ones for nvidia and AMD graphics. Also, it is not an easy repair, as you need to remove other parts before you can reach the cooling fans.

Unless you're a skilled laptop tech, it is recommended to let an expert do it. Either a Dell out-of-warranty repair or an other skilled technician.

Thank you for the video - I purchased my Dell Precision 7520 with the NVIDIA graphics less than a year ago & have exactly the symptoms in your video. One possible difference is your video seems to show the sound coming from the right rear while my sound is coming from the left rear. It sounds like someone out of breath & huffing & puffing. I'm out of warranty but searching for this problem makes it clear many have this problem. I hope Dell will step up here and do something to help all of us affected by the fan issue.

Addendum - I did run the boot hardware diagnostics & the result was 'all good'. Another issue - I have allowed the Dell Support Assist to install updates - it is stuck on the BIOS update and cannot update past version 1.19.2. And I have been unsuccessful updating manually (using a USB stick)

Is it the right-hand side fan or left?

@TheQuickFox​  thank you for the video and the answer. Did the service center staff tell you what was the cause of the malfunction ?