Identify where combustion air comes from
Combustion air is the air a fuel-burning appliance uses to burn fuel. Some installed arrangements draw it from indoors; others bring it from outside. For example, the Lennox ML296DFVK manual distinguishes outdoor and indoor combustion-air paths by installation type. Your installation governs the requirements. Lennox combustion-air example.
Seeing a plastic pipe does not establish that every appliance in the closet is isolated from room air. A water heater or another appliance may have separate needs. Ask a qualified professional to evaluate the shared space before changing a door, grille, or enclosure.
Trane's owner manual warns against blocking combustion and ventilation air. Trane air-opening requirement.
Find out what the sound-control work should solve
Ask whether the noise originates in a component, airflow, or vibration transmitted through the installation. If a fault or unsuitable operating setup is found, address it first. A soundproofing project should not be used to hide a new ignition noise or mechanical problem.
If enclosure changes remain appropriate, ask for the design to preserve required openings, material suitability, clearances, and access. Have the professional identify whose approval and work are needed. Acoustic or construction work may require another provider outside MGI's confirmed offering.
Do not wrap the furnace, place foam against it, cover a louver, or store blankets in the closet. Do not turn down internal blower settings to make the enclosure easier to quiet. Keep the existing required openings and service space clear while the plan is being checked.
Judge the complete result
Ask what improvement is expected in the affected room and how the result will be compared. Separate the cost of equipment correction from any optional acoustic work. A promise that a door change will eliminate all furnace noise needs more support than a product description.
Your next step is a qualified review of the closet and noise source before buying materials. That gives you a sound-control plan that respects the equipment's operating needs and the way you use the room.