A bit of dust inside a laptop can see the fan blowing hard almost constantly. It only takes six months or so for this dust to accumulate and have a noticeable impact on the loudness of the laptop. The problem may not be noticeable from the outside.
But if the fan seems noisy, it's worth taking a look inside. Google should provide instructions; with mine it's just a case of unscrewing the CPU panel on the back. You'll find some copper heat ducts and a fan blowing out through the copper grills of the heat sink.
A closer inspections reveals how dusty the fan is.
Now is probably the time to look up a professional cleaning guide. Cleaning the fan with an artist's paint brush and then giving it a good blow like I do is probably not a good idea. However you clean the fan, keep a finger on the central part to prevent the fan spinning, as that can damage the fan.
I like to clean the heat sink fins with a small strip of card.
Holding the laptop up to the light reveals nice clean copper fins.
If yours doesn't, maybe you should have followed the advice about using a professional cleaning guide.
Keen eyes may spot that I have broken off the bars of the case grill that used to cover the copper heat sink grills. I did this one summer when temperatures where I was living were in the low 30's Celcius. The plastic bars prevented me cleaning some of the slots in the copper grill, and also restricted air flow. Breaking them off noticeably reduced fan noise in hot weather. As the laptop is only used on a desktop, possible damage to the copper grill was not an issue.