weed
耘 centers on removing unwanted plants from cultivated land: weeding. It is a specific agricultural action rather than a broad concept.
耘 combines 耒 (a farming tool) with 云, which likely contributes the sound. The character originally referred to weeding with a tool.
The left side 耒 is a farming tool, and the right side 云 looks like a cloud. Imagine using a tool to weed out clouds of unwanted plants from a field.
For ウン, picture weeding under a cloudy sky and saying 'un' as you pull each weed: un -> ウン, the sound of effort while clearing the field.