See http://www.williambennettflute.com/rampal_dufrene.htm where the English flutist William Bennett cites this particular characteristic of Rampal’s playing as a point of skill he wished to improve when he sought out Rampal in Paris for tuition; Bennett also speaks about this in the 1983 BBC Radio 4 profile of 'Rampal - Prince of Flute-Players' where Rampal himself surmises that a native French-speaker's natural facility for producing the "tu" sound in speech was one reason why the articulation of many French flute-players was so good.
