Venetian (the Italo-Romance language spoken in the area of Italy roughly corresponding to the Veneto region) has a weird sound which is usually called l evanescente (evanescent l). It varies geographically (from a completely normal /l/ to mute), but its most distinctive form has been described by Lepschy in Fonematica veneziana (pg 18-19) as (my translation)
an articulation where the air passes through a depression in the central part of the top of the tongue, lifted towards the palate, while the two sides of the top of the tongue are in contact with the sides of the crown of the upper teeth
The pronunciation of toła by ness1 in Forvo provides a fairly characteristic realization.
Ferguson, in his A Linguistic History of Venice claims it has no IPA representation, so I assume it is a rare sound. He also reports it has been described as «an unrounded prevelar semi-vowel».
Q: Is there any other language containing this sound?