[Sailariel]

I'm thinking a glazed belt and/or glazed pulleys. Certainly, some belt dressing directed at the pulleys and the belt would not hurt. If that doesn't solve the problem, a stethoscope like Mark suggested, would go a long way in diagnosing the problem. Back in the 60s, when a lot of us were riding Brit bikes, we would use a stethoscope and listen to the valves after they were properly adjusted to get a baseline as to how they should sound. Then, periodically we would give a listen, and if they sounded different, we would adjust them. Life was so much simpler then.