At 30K on the WR G2s on the XC, I haven't noticed any noise increase. They still ride quiet and smooth.
I've got 20K on the same tires on the MB, and they're still quiet and smooth...
So many factors affect tire wear and noise, it's hard to determine the precise reason they've gotten noisy in your cases...so far, so good in mine...
2006 Corolla (kid's car - 165K, Eco Contacts, Castrol Edge 5W30)
2005 MB S600 (84K, Michelin AS3, M1 0W40)
2002 V70-XC (201K, AltiMax RT43, Pennzoil Ultra 5W40)
2002 V70-T5 (179K, IPD bars, Bilsteins, Conti PureContact, Mobil 1 0W40)
1992 MB 300E (Sportline - 160K, Yokohama Avid, Castrol Edge 0W40)
1990 4-Runner (V6, 4WD, mild lift - 257K, 31" BFG AT KO, Castrol 10W40)
1932 Packard Sedan (straight 8, dual sidemount spares, original paint and interior, Shell Rotella 15W40)