A "Loud" heist is a style of heist in Grand Theft Auto V.
This style involves barging into the place weapons drawn
This style is fast and effective, but makes the most noise and police might be alerted and show up fast
Compare this to the Smart style of heist, where for example the robbers disguise themselves as Pest Exterminators and use a Bugstars Van as cover to cleverly gain access to the location.