You have an unsealed vent.
Floor vent seal.
If you do unscrew one of the floor vent covers and set it aside.
If you live in a house with a crawl space there is a good chance the floor vents are not sealed.
The goal is to prevent conditioned air from escaping or unconditioned air from entering the thermal envelope.
Inspect your floor vents.
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Look into the vent right where the ductwork meets the floor.
Check to make sure you have a crawl space and that you have air vents coming up through the floor.
To properly seal around air vents apply caulk or expanding foam between the ducting and the sheetrock.
This is one of those things that can go unnoticed but have.
The good news is that an alternative method of cleaning and sealing air ducts exists.
Is there a gap between the metal duct and the floor.