The plenum.
The box where the ducts meet the equipment. A leak here is the most expensive leak in the system.
A plenum is the box that connects the air handler to the duct system. There are two.
The supply plenum sits on the output side, immediately after the evaporator coil. All the conditioned air passes through it before branching off into individual runs.
The return plenum is on the input side, where return duct delivers house air back to the blower and the filter.
Why a leak here is the worst one
Every cubic foot the system moves passes through the plenums. A leak in a branch duct loses the air serving one room. A leak in the supply plenum loses a share of everything, before it has gone anywhere.
In a Phoenix attic that means dumping air you just paid to cool into a space well over 150 degrees. A return plenum leak is worse still: it pulls that 150 degree attic air into the system, which then has to cool it from there.
That shows up as a system running constantly, a house that never quite gets comfortable, and a bill nobody can explain. Nothing has failed, so nobody investigates.
What goes wrong
Tape failure at the seams. Duct tape, as sold, does not survive decades in an attic. Mastic does.
Separation where the plenum meets the unit, from vibration or from the unit shifting on its platform.
Crushed or deformed plenums, usually from somebody kneeling on one.
Undersized return plenums, which is a design issue rather than a fault and shows up as high static pressure and weak airflow everywhere at once.
Why it comes up in a diagnosis
Static pressure readings are taken across the system, and plenum condition is part of what explains them. A system with a clean filter and high static has a restriction, and an undersized or collapsed return plenum is a frequent answer.
Sealing and duct modification work is off the published list here, so if that is where it lands you get the measurements and a referral rather than an estimate.