Hi Charles,
It's usually just one of the little nylon pulleys in the tape counter drive setup crying for some lube. A good clean and lube will usually fix it. If you decide to remove the tape counter drive belt to clean it or replace it as well, a tip is to fish it out through the gap around the front of the reversing idler using a probe with a hook on the end. Tech Shawn has a little set of what are more or less pointed scribers with various bends at the ends. They come in very handy for fishing belts and wires out of the innards of a tape recorder. The alternative is to pull the capstan motor so you can get at the belt from behind. That isn't a terrible job, but kind of a lot of fooling around for a dadgummed belt.