A couple of years ago my contractor replaced my old A/C only 2 ton 10 SEER York with a 14/15 SEER 2.5T Goodman with variable Air Handler (16x20). Each of 4 bedrooms has it's own 8x8 return, except the master BR which is 12x12. Since that time, the return 12x12 in the master bedroom howls as if the unit is starved for air or velocity is too high. None of the returns in the smaller bedrooms make any noise or are blocked. None of the duct work was changed when the unit was changed. I understand you can't have to much return and I'd like to add another 12x12. My question: I'd like to avoid cutting sheet metal to tap into the return plenum. Can I put a metal T into the 12" line coming from the 12x12 and put another 12x12 in the MBR? IOW will the 12" flexduct handle 2 12x12 returns and cut down the noise? So I'd have 2 12x12 returns joined through 12" flex. It would be about 8' between returns in the MBR. From what I can see 12" will pass 600CFM.
The other idea was to cut into the metal return duct and run a separate 12" return. It's just that the space is difficult to work in and cutting in the T would be a piece of cake.
I hope I'm making this clear. My thanks to anyone responding.