If your are refering to the plenum being a large box say for air mixing the answer is no. you should have filter in return side however be sure you have access.

A plenum is an air chamber box at both the inlet and outlet of an air conditioner. It is the box that channels the air from the air conditioner to the ductwork. IE: supply air plenum; return air plenum.

Plenum rated cabling is required when used in an open ceiling return air environment. That is when the return air for the hvac units use the space between the ceiling of the first floor and the floor of the second floor as a return air plenum. The way to tell is if the return air vent has a duct attached to it. The duct will run to the air conditioning air handler. This condition IS NOT a plenum rated ceiling. However, if the return air vent does not have a duct, that means the air conditioning air handler is using the air space between floors as a plenum. This condition requires the use of plenum rated cabling. If plenum rated cable is exposed to flame, it does not give off toxic gases as it melts.

The Trox plenum box radial air discharge ensure a high degree of comfort.

The blower motor resistor is located on the right hand side of air inlet(plenum). Where is the air inlet? Where is the (plenum)?