Typically with newer installs we line the main plenum off the top of the furnace to help with noise, as well as the return drop with sound dampening material. This is either fiberglass acoustical duct liner or rubber acoustical liner
since this is an old install something else would be more appropriate . I would consider taking a 10” pipe off that first picture out 10’ , or to save 2” headspace run 12x8 rectangular. If you want to increase that a little and line that with sound material.
just separating it from the system a little will help a lot, as well as eliminating the pipes off the cap.. but the way the whole system was installed it may now just push that air somewhere else because of lack of back Pressure. If it is a small enough house it may cost 2-4K to redo, with a brand new insulated system
it would suck to fix the one problem only to create a new one somewhere else