Q:

Does the vowel combination /ou/ appear in any Spanish words?

Spanish has many diphthongs, but I have never noticed 'ou' appearing in a word except for bou (a Catalan loanword), the surname Bousoño, and the Kantian term noúmeno.
The sound does occur across word boundaries - "tengo una casa" etc - and in some compound words e.g. estadounidenses, genitourinario, finoúgrio.
Are there any other Spanish words that contain the combination of vowels /ou/ (as a diphthong or in hiatus)?

Note: I'm not including un-assimilated French/English loanwords like coulis, rouge, touche, roulotte, bourbon, house etc as they tend to be pronounced similarly to the original voice i.e. /u/ or /au/ e.g. in sioux and its assimilated form siux, or in output.

Source:

Bowen and Stockwell do not list /ow/ (written /ou/ in this note) among their syllabic nuclei. It occurs in admittedly rare forms such as bou 'fishing with a net dragged by two boats' and the proper name Bousoño.
 • "A Note on Spanish Semivowels", Sol Saporta (1956)

A:

Realizando una búsqueda para palabras que contienen "ou" en el DLE, se genera la siguiente lista: 

bou, proustiano, soul1
estadounidense, estadounidismo, genitourinario
art nouveau, boutade, boutique, coulis, gouache, gourmet, mousse, rouge, roulotte, souvenir, tour, tour de force, tournée, troupe 
choucroute (chucrut), glamour (glamur), coulomb (culombio), goulash (gulasch), joule (julio), sioux (siux), soufflé (suflé)
bourbon, boy scout, chill out1, country1, house1, output, underground. 

La búsqueda para palabras que contienen "oú" solo arroja dos resultados:  

finoúgrio (también fino-ugrio)
noúmeno 

Como se puede ver, son todo extranjerismos (en cursiva - sin cursiva la adaptación española) o compuestos (como estadounidense, finoúngrio), por lo que además de los casos contemplados en la pregunta (bou y noúmeno), únicamente proustiano y soul (como señala acertadamente walen) cumplen los requisitos pedidos.

1. Estilos de música.

