Sounds like the spindle bearings are dry and about to lock up. I can disassemble the spindles on my mower and replace both bearing for under $4.00 so remove the deck, remove the belt and try turning the spindles by hand and listen/feel for one or more of them to make a grinding sound/feeling when turning by hand. Rock the pulleys side to side by hand and feel for excessive play. If you find one of the spindle bearings to be defective, most likely the others are not far behind and it would be prudent to replace all of them at the same time. If you can disassemble the spindles and get the bearings out you should be able to measure and match them up with you local supplier or find them on the web. This would also be a good time to sharpen and balance your blades.
