new rear squeak (not rear ball joints)

[THE VEIN]

I already have the rear ball joint squeak on both sides on my car but now I'm starting to notice a new squeak that sounds like it's coming from under the passenger side rear seats. I can only hear the ball joint squeak when I have the windows down, but this new squeak is very loud and I can hear it no matter what. I'm thinking is the subframe bushing but is there any other way I can check if it's the subframe bushing or maybe it's the rear ball joint getting louder