always an interesting question .... since mine has done this for years too. i'm from the older school that suggests it's better to have the belts too loose than wear out the bearings. it's always worth looking for problems or changes but having tried most of the same fixes including 'belt dressing', i'm pretty convinced that the electrical system is doing just exactly what it was designed to do and i'm simply unwilling to tighten the belt enough. i have not broken any belts and when i do replace them occasionally they are less squealy for a while. at 326k it is distracting but everything seems to keep working. i remember it was worst when the main oil seal was weeping but that went away after a timing belt / seal change.