Any time I input any melodic note, that instrument plays back along with a marching bass drum sound that the midi note is linked to.  Hope that makes sense.  Not sure what is going on. 

Mac 13inch Late 2011
16GB RAM
2.4ghz processor
Sibelius 7.5.1
VDL 2.5.5
TWS Template Sib7.0a

Is it possible the MIDI channel you are using for the melodic instrument is the same channel the bass drum sample is on? You might want to add an instance of Kontakt and see if it corrects itself.