No idea how to replicate it, but two games in a row was facing default skin Morde. Now, correct me if I'm wrong but the Pentakill Morde has a guitar noise right? Like someone strummed the guitar and slid their hand down the neck?
So in both games while either hitting or killing Morde I heard this noise. Maybe I'm just crazy but, hearing it 4 or 5 times is good enough for me to think I'm not.
Cass, Xin, Teemo, Singed, Tryn
Kennen, Jax ,Tryn, Morde, Someone
Cass, Singed, Tryn, xin, Malz
Morde, Taric, Nocturn, Lux, Zilean.
These are from memory, so first game might not be 100% accurate on either team.
Not sure if team comp matters but I figure more details is nice.
It seemed like it was happening more when he was dying but I can't be positive of that.