Article: Unique to the development of Dutch is the collaps of older ol/ul/al + dental into ol + dental, followed by vocalisation of pre-consonantal /l/ and after a short vowel, creating the diphthong /ɑu/ e.g., Dutch goud, zout and bout corresponds with Low German Gold, Solt, Bolt; German Gold, Salz, Balt and English gold, salt, bold. This is the most common diphthong along with /ɛi œy/. All three are commonly the only ones considered unique phonemes in Dutch. The tendency for native English speakers is to pronounce Dutch names with /ɛi/ (written as ij or ei) as /aɪ/, (like the English vowel y) which does not normally lead to confusion among native listeners, since in a number of dialects (e.g. in Amsterdam) the same pronunciation is heard.

Now answer this question: What's the term we use for the type of sound made for two vowels in words like the Dutch "goud" or "zout"?
diphthong