It is Charumera tune, famously known to indicate the presence of ramen food car(t).
Charumera is a double-reeded musical instrument that originated from China, known as sunoa, similar to the oboe. (Not to be confused with a ramen brand with the same name by Myojo/Nissin and a PS1 game with the same name). The name charumera itself was derived from the Portuguese word charamela, a similar but different musical instrument brought by them when they visited Japan during the beginning of the Edo period.
According to Yamaha - Trivia : The charumera was the oboe's cousin
The charumera was a woodwind instrument that was played by people running ramen stands in the not-too-distant past.
According to Archive.org - Charumera (PS1 Game)
the word “charumera” is actually derived from the Portuguese “charamela”, a type of flute that was brought to Japan around 400 years ago. This flute has traditionally been used by yatai (ramen mobile food stalls, [...]
According to J A P A N o F I L E S # 8 – Chronicle of Starting a Shina Soba Shop,
The name ‘charumera’ refers to a double-reed woodwind instrument that is said to have derived from ‘charamela’ – Portuguese for shawm -, hinting at a possible introduction to the Japanese by way of Christian missionaries from that provenance sometime during the 16th century. The simplicity of the instrument made it popular among pushcart owners, not only because its sound signalled their impending arrival, but also due to the clever employment of signature melodies in what can only be described as a primordial manifestation of brand awareness. All these practices are exquisitely demonstrated in a scene from Yasujirō Ozu’s 1936 film Hitori Musuko (The Only Son). Needless to say, a viewing of the entire picture is most advised.
Some videos demonstrating the action: