The sun makes no sound, so technically, there can be no onomatopoeic words for "sun." However, you can imagine the sun making sounds, such as the popping out of solar flares, swirling and rushing...
Not really; especially when used by itself. The word "noisy" has the sonorant n and z sounds in it, and two ee sounds. The word brings to mind the sound of murmuring more than actual loud noise....