is there a filter in this return grill ? What is your main return line duct size?
Assuming there is, your way undersized.
You have enough return for a 2.5 ton system.
20x30 is good up to 1074 CFM
You would probably benifit from adding another 20x30 somewhere in the house.
Does your return air grill sound like an industrial vacuum cleaner?
Here is a helpful document: