Using @MrHen's answer as a springboard, I jumped around Wikipedia's phonetic entries some and came across liquid consonants, of which English has two, /l/ and /r/. There's this on the etymology of the term:
The grammarian Dionysius Thrax used the Greek word ὑγρος (hugros, "moist") to describe the /l,r,m,n/ phonemes of classical Greek.[2] Most commentators assume that this referred to their "slippery" effect on meter in classical Greek verse when they occur as the second member of a consonant cluster.[2] This word was calqued into Latin as liquidus, whence it has been retained in the Western European phonetic tradition.
Apart from the technical definition of the term here, I like liquid as a possible answer to my question, all the more so because it is actually used in a phonetical context.