I have several resources and dictionaries that suggest the word 透 (tou) is pictophonetic where 辶 (chuo) suggests the meaning and 秀（xiu) provides the sound. This confuses me because 透 and 秀 do not seem to be related in sound. I assumed that this may be due to a classical chinese pronunciation but was unable to verify that or find more information.
What is the correct etymology here? thanks!