Belts slippping/squeeking on Hesston 946

 I have a 946 silage special and am experiencing an unnerving squeek, shriek, squall, or otherwise brief, high pitched noise at irregular intervals after the bale size has exceeded 50 inches, predominately when baling dry hay. I have identified the noise to the belts briefly slipping on the upper, rubber coated drive roll when any sudden loading, e.g. a wad or slug, is encountered. My hydraulic pressure is running 2500psi, hydraulic system is not overheating, cylinders are all reasonably close in temperature. The squeek will drive you crazy in tough hay on in clumpy windrows. I was wondering if anyone has experience solving this problem? My baler has about 6500 bales on it with original belts. Thanks

Belts slippping/squeeking on Hesston 946