Thank you for the answers. Your explanation about the trap was helpful. I'll leave those alone.
Each unit has two returns - each of these used to hold a 14x20 filter (now the filter is a 20x25x4 right at the furnace). I'll have to check to see if the grill itself is vibrating. Certainly seems possible although the "sound" really seems to be coming from the unit itself. You don't really hear it standing next to the unit but you hear it standing in the house right below the unit.
This website is extremely helpful. Thank you to all who contribute.