If you contract with someone, be sure they specialize in Duct Cleaning.
I recommend a particular Duct Cleaning Company in my locale because ive
seen how they do the job : They use special brushes that whip around
inside the rectangular trunk ducts and round branch ducts about every
6-8 feet , while a huge Suction Machine on a trailer is sucking thru a
12" round flexible duct connected to the supply/return air ductwork.
(They drill 1" holes on the underside of the ducts and cover them with
flush-mounted plugs when done). They pull the blower assembly out of the
furnace and clean it/oil motor , as well as the blower compartment of
the furnace . They remove all supply air/return air registers/grilles
and wash them , and also use special brushes with Suction Machine down
each opening . They start out with an empty Suction Machine reseviour
that they show the customer before starting, and then after the process
is completed. They average : 14-18 lbs. of dust/dirt/debris from a duct
system. To be sure they did an adequate job, they put a remote
controlled camera inside the main ducts that rolls along giving a video
on a screen for the customer to see. Finally, if the customer has
severe allergies to mold..they inject a mold killing agent into the duct
system for an additional nominal fee. For a 2000 sq.ft home , they
typically charge about $400 and it takes 2 men 4 hours to complete.

Conversely, there are 'duct cleaners' who use Leaf Blowers shooting air
down each register for $99.95 .