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Why a kitchen extract fan gets louder over time
Extract noise almost always builds slowly, and the cause is nearly always mechanical rather than electrical. This is what changes inside a fan and a duct run over a few years, and what a kitchen manager can check before booking anybody to attend.
Extract noise is usually mechanical, and it usually arrives slowly enough that nobody notices the day it started.
What actually changes
Three things move over the life of a system. Bearings dry out, so a fan that ran quietly for two years starts to whine before it starts to rumble. The impeller collects grease unevenly, the wheel goes out of balance, and the whole housing shakes. Anti vibration mounts compress and harden, and once they do the fan is effectively bolted to the structure, so the building carries the sound.
What to check before booking anybody
Turn the fan off at the isolator and listen to the kitchen without it. If what you heard was drumming rather than whining, look for a duct panel that has worked loose.
Check the filters. A blocked filter bank makes the fan work against a restriction, and a restricted fan is a loud one.
Note when the noise appears. A fan that is quiet on start up and loud an hour later is a heat problem rather than a balance problem.
When it needs an engineer
Bearing noise does not improve on its own, and a fan left running on failed bearings takes the motor with it. Booking while it is still a service is cheaper than booking once it is a replacement.