MacBook Pro fan always on!

My MacBook Pro sounds like a mini wind turbine. It’s nearly always on, and gets triggered by things that use little to no CPU power like simple internet browsing. My laptop doesn’t even have to be hot for the fan to activate, and is in fact not hot most of the time while the fan is doing its thing. Also its been happening since late 2016 so this isn‘t something that just happened. It’s very annoying and surprisingly loud.

Is there any way to fix this? I have tried looking at Activity Monitor but the only thing I always see taking up a lot of space is Safari and the occasional Firefox (zzzzz). I have also tried resetting the SMC to no avail. I understand my laptop is an older model so would downgrading to an older OS help the situation? These are my specs:

MacBook Pro (Retina, 13-inch, Early 2013)

Processor: 2.6 GHz Intel Core i5

Memory: 8 GB 1600 MHz DDR3

Graphics: Intel HD Graphics 4000 1536 MB

Would appreciate any help!