Scott,
the sliding noise is a different multisample, triggered when the midi velocity (i.e. the strength of the finger hitting the note) reaches a given value (if I remember correctly, in this case is something in the 120-125 range); I know this for sure because I have sampled all my favorite sounds from the Tyros 2 and am presently using them in my Korg PA2X Pro.
Since in the Tyros 2 there is no way to edit the crossover between different layers of a sound (like the Warm Solid Guitar), my opinion is that you have three options:
1- either you choose a different sound from the Guitar bank (the Tyros 2 has many variations of the same basic sample, with or without added noises)
2- or you modify the global velocity curve of the keyboard choosing a "harder" set, so that even if you hit the notes really hard you will never reach the treshold velocity required to trigger the sliding noise.
3- (but this is a semi-serious suggestion

) simply play the notes in a softer way.
Hope this helps.