A MacBook that roars while you are doing nothing more demanding than reading email has one of four causes. Three of them are cheap to fix.
Check software first, it costs nothing
Open Activity Monitor and sort by CPU. If one process is sitting at 100 percent or more, that is your answer — often a stuck browser tab, an indexing process that never finished, or a sync client caught in a loop. Quit it and the fan settles within a minute. If nothing is above a few percent and the fan is still loud, the problem is physical.
Dust in the fins
Air is pulled through the hinge and pushed out through a stack of very fine heatsink fins. Over two or three years in a city like Hyderabad those fins clog with felted dust until almost no air moves. The fan compensates by spinning flat out and the machine still throttles. A proper strip-down and clean fixes it completely.
Dried thermal paste
The paste between the processor and the heatsink dries and cracks with age, so heat stops crossing into the heatsink at all. The chip runs hot, the fan panics, and performance drops — often by a third or more on older machines. Renewing paste and thermal pads is routine service work.
A failing fan or sensor
Less common, but it happens: a worn bearing makes the noise without moving much air, or a failed temperature sensor makes the system assume the worst and run the fan at full speed permanently. Both are straightforward replacements.
Why it is worth fixing rather than living with
Sustained heat is what kills laptops. It ages the battery faster, cooks the paste further, and over years stresses solder joints on the board. A thermal service is one of the cheapest things you can do to extend the life of a Mac — and it makes the machine noticeably faster at the same time.