^^ those would be my best three guesses as well. #1 and #2 can be fixed easily, #3 is pretty much just a 'life-with-it' syndrome of our cars. You can buy a IRs mem brace from Kenny Brown, that might help things out, although I've never used one before.
I had this problem in my 98 GT, I'm actually getting it fixed tomorrow. But anyway, for me it was a 'whooshing' noise, and it ended up being my rear axel bearings going bad. I dont know anything about you IRS guys, but maybe its that? I'm clueless with IRS