The retroflex trill is a sound that has been reported from the Dravidian languageToda, and confirmed with laboratory measurements. Peter Ladefoged transcribes it with the IPA symbol normally associated with the retroflex flap, ⟨ɽ⟩. Although the tongue starts out in a sub-apical retroflex position, trilling involves the tip of the tongue and causes it to move forward to the alveolar ridge; this means that the retroflex trill gives a preceding vowel retroflex coloration the way other retroflex consonants do, but the vibration itself is not much different from an alveolar trill. Thus, the narrower transcription ⟨ɽ͡r⟩ is also appropriate.
Wahgi has a similar trilled allophone of its lateral flap, [̥r̥], but it is voiceless.
Wintu and Lardil are other languages with a reported (apico-)retroflex trill where the tongue apex "approaches" the hard palate (this is not sub-apical as in Toda). The trill has a retroflex flap allophone occurring in intervocalic position.
Variations of the retroflex trill in IPA symbols.
Several languages have been reported to have trilled retroflex affricates such as [ɳɖ͡ɽ̝] and [ʈ͡ɽ̝̊], including Mapudungun, Malagasy, and Fijian. However, the exact articulation is seldom clear from the descriptions. In Fijian, for example, further investigation revealed that the sound (written ⟨dr⟩) is seldom trilled, usually realized as a postalveolar stop [n̠d̠] instead. In Mapudungun, the sound (written tr) is strongly retroflex, causing /l/ and /r/ following the subsequent vowel to become retroflex as well. In the southern dialect it varies between /ʈɽ/ and /ʈʂ/, but it is not clear whether the letter ⟨ɽ⟩ represents a trill or a non-sibilant fricative.
Goeman, Ton; Van de Velde, Hans (2001), "Co-occurrence constraints on /r/ and /ɣ/ in Dutch dialects", in van de Velde, Hans; van Hout, Roeland, 'r-atics, Brussels: Etudes & Travaux, pp. 91–112, ISSN0777-3692