American English Phonetics

9.2.1 Approximant Devoicing

Like vowels, the GA approximants /l,r,j,w/ can follow syllable-initial /p,t,k/, so that the voice delay that occurs after the release of fortis stops may also affect a following approximant.
Examples would be pure and twin.

The effect is particularly striking in the case of /tr/, which is usually pronounced as a completely voiceless affricate. In words like pure and twin, /j/ and /w/ tend to be partly devoiced: [pj̥ʊr, tw̥ɪn]. Note that the devoicing of /j/ in GA pure is paralleled by that of AN /j/ in the diminutive ending and the personal pronoun je, as in hapje, heb je, koekje.

The devoicing of approximants automatically leads to the generation of (voiceless) friction, as can easily be heard and felt when you slow down AN hapje to [ɦɑpçːːjə]. Exactly the same goes for devoiced GA /l,rj,w/.