Won't cure it long term, but some keen plant growers have a thing that sprays water on a plant's leaves. Spray the belt with the engine running - if it's that, silence will very quickly be effected.
It'll wear off just as quickly, but an industrial ironmongers will have some spray on belt dressing. Or you could mug a passing violinist as the belt dressing is usually basically just aerosol rosin (in solid form, applied to violin bows).
