We had all sorts of belt noise problems. A quick & dirty way to silence a belt is Softscrub. Put it on the belt with the engine off, and start it with the hood closed because it will spatter all over the place. This will silence any belt noise quickly. The downside to this method is it makes a huge mess that is hard to clean up afterward. The upside is you'll know immediately if it's the belt making the noise or something else that could be more serious.
Being that only two things have changed since the noise started, it's either the belt or your transmission. Like others have suggested, taking the belt off is more preferred to the method I described, but I put it out there as a really quick way to eliminate belt noise from the equation if you'd rather try it that way. Just be warned: it surely does make a mess!
As a side note, Softscrub silenced a chirpy belt for months for us when everything else failed. It's a quick & messy (but effective) temporary fix.
-1999 Plymouth Grand Voyager SE. Factory Emerald Green, with aftermarket rust accents ("What rocker panels?!") 192,000 miles (and counting) with the 3.8L and ORIGINAL 41TE. The Spinach-Van!
-1996 Plymouth Grand Voyager SE, Factory Rosewood paint with ZERO rust, 3.3l, 175,000 miles. The "new" van. It's a baby. The Eggplant-Van!