Hi @metachii ,
It may be dust but it could also be a faulty fan.
Here’s a link to the service manual for your laptop.
Scroll to p.71 to view the necessary pre-requisite steps and then the procedure to remove the heat sink/fan assembly. (depending on the specifications of your laptop, you may be able to remove the fans without removing the entire heat sink assembly).
This should allow you to check if it is dust or a faulty fan (bearing perhaps?)
If you need to replace the fan or heatsink fan just search online for Dell Alienware 15 R3 fan to get results for suppliers.
If the laptop is still under warranty it may be better to consult Dell regarding the problem as you don’t want to void the warranty by opening it yourself. You may have reason to use the warranty in the future ;-)