Follow the comfort pattern
If one bedroom stays warm, an upstairs area struggles, or doors move when the system runs, note which rooms and conditions are involved.
Look for evidence
Whistling at a grille, loose insulation, disconnected flex duct, dusty marks around a joint, and a return pulling attic air are useful clues. Do not enter an unsafe attic.
Balance the whole system
Sealing one visible joint may help, but a good evaluation also considers return airflow, supply balance, insulation, equipment capacity, and static pressure.
Do not trade comfort for noise
Closing vents can change pressure without fixing the distribution problem. It may make one room feel different while increasing stress on the blower or leaving other rooms worse.
Ask for before-and-after evidence
A useful duct recommendation explains what is leaking or restricted and how the change should affect airflow, temperature, pressure, and operating cost.