Q: Why is my Mac's fan constantly running even when I'm barely doing anything?

Download and post an Etrecheck report:

http://www.etresoft.com/etrecheck

Run an Apple Hardware Test:

https://us-mg5.mail.yahoo.com/neo/launch?.rand=egb7pucia3r8v

Note that an error free AHT is not conclusive.

Install a temerature/fan speed application.

Set Activity Monitor to ALL PROCESSES and CPU to display values from high to low.

When the fan speeds up, post images of both for review.

Ciao.

Please read Apps can affect Mac performance, battery runtime, temperature, and fan activity - Apple Support, particularly the information contained in the links that follow "Learn more".

ctyw wrote:

 

Oh, and also to add, my Mac has also been receiving countless popup ads and TurboAds all over the pages. Checked my extensions and can't seem to figure out the big "Why" for this one either.

Please read If Safari is slow, stops responding, quits unexpectedly, or has other issues - Apple Support, but the relevant excerpt in your case is likely to be the following:

If the issue is unwanted pop-up windows, ads, and graphics while surfing the web, learn how to remove ad-injection software (adware) from your Mac.

If it turns out that your Mac has been affected by adware, please understand that only you can prevent that from happening. To learn how to recognize adware so that you don't install it to begin with, please read How to install adware.

Upgrading OS X to the latest release and keeping it up to date will automatically remove most popular adware variants. Read update to the most recent version of OS X El Capitan.