As my opinion, ur patient problem results from downward rotation of scapular. If ur patient has weakness in upper trapezius because of bad habitual posture (i.e forwarded neck), ur patient also has stiffness in levator scapuar and rhoboid major and minor.
and Latissimus dorsi olso can cause downward rotation of scapular
but snapping sound in retration means it is more likely due to levator scapular.
so i hope u do eaxmine ur patient levator scapular and rhoboid and if u find stiffness or shortness in these muscle , doing massage or applying kinesio tape would be helpful....