In the four years I've owned my 98 5.4 SMB, I've lost track of how many belts I've put on it to remedy squeal and/or chatter. I've replaced all the pulleys and the tensioner. Have tried OEM Motorcraft and various aftermarket belts. Everything seems aligned. The chatter is not bad, really, but I'm so attuned to it that it makes me crazy.
Before this last trip (~9,000 miles) I put on a Goodyear "Gatorback" belt (now under the Continental brand, see http://www.thequietbelt.com
), which doesn't use ribs but instead a checkered pattern on the drive side. Lots of posts on the web from other 5.4 owners rave about. So far I haven't had any noise at all from it, so I'm hoping this may be the solution. It's not much more expensive than other belts, either.
On a positive note, I'm VERY good and fast at replacing the belt on my van now! :-)