In Cantonese, it's "fun see". "fun" means rice noodle and "see" means silky thread.
Thanks for Lee for the info. But does "fun see" also cover noodles made of mung beans? I know mung bean noodles are used in Chinese food, because here in Saudi Arabia, supermarkets stock these noodles either from Taiwan or Thailand. In Filipino they are known as "sotanghon".
I suspect Thai word wun sen comes from a Chinese dialect. A lot of the Chinese food words have gone into Thai and were Thaiised.