Gear: Late 2013 27" iMac, 16GB RAM, 3.2 GHz 1600 MHz DDR3, using USB mouse/keyboard in preference to supplied wireless items, internal audio interface, no external drive. Logic Pro X 10.2.1. Yosemite 10.10.5. Problem: Experiencing very VERY faint "ch ch ch" sound at end (release) of midi notes in virtual instruments string section in orchestral arrangement. File imported as midi from notation program. Have been unable to identify settings in notation programs that might be causing problem. Solutions attempted to date: Tried different virtual instruments; Tried different notation program: Disconnected mouse/keyboard (faulty mouse caused similar effect some time ago under different circumstances); Tried fiddling with plug-ins, especially drive and compression; Tried removing legato overlaps (end of notes slightly overlapping start of following notes – I had also used the force legato facility previously); The problem is also apparent on other orchestral arrangements regardless of whether or not I listen through built-in speakers, Sony near field two-way ported monitors (holes plugged with old socks) or headphones. If I bounce to MP3 the effect is still there. Thank you, John Morton.