Dictionaries suggest 透 (tou) is pictophonetic where 秀 (xiu) provides the sound. Is this through a classical chinese pronunciation? 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!
Pronoucing them in Cantonese will make sense to you.

透，tau3
秀，sau3