This sounds like Sycamore Aphid activity. These infest buds and the underside of leaves of sycamores and other Acers in spring and summer. Large quantities of 'honeydew' are produced, fouling foliage, becoming sticky and shiny in dry weather.
Impossible to control this aphid on large trees, but small trees will benefit from a tar-oil winter wash in early January to kill over-wintering aphid eggs. Later in the year, spraying with a non-systemic insecticide will give up to 2 weeks protection, so will have to be done regularly. Look for these ingredients on the label. Diazinon, pirimicarb, fenitrothion, HCH, pirimiphos-methyl. Hope this helps.