[soloan]

Hi. I am using Protools 5.1, TDM 24 mix plus system on Mac OS 9.2. I've just experienced a problem on Fading. Here is how it happened:
I have imported several sound files by "Import Audio to Tracks". Some sound files are SDII 24/48 while the others are wav 24/48. The protools session is SDII 24/48. When i import those wav files into the SDII session, I didn't convert them to SDII. I just leave them as is. No problems on mixing until:

I grouped several tracks together, including SDII and wav, and I fade out this whole group by pulling down the VOLUME FADER. During my manual fading on a Mackie HUI, it sounds fine. BUT WHEN I LISTEN TO THE PLAYBACK, IT'S GOT A CLICK SOUND IN THE MIDDLE OF THE FADE. I checked all the tracks' waveform. There is no clicking on those grouped tracks and no edit point either.

What's the problem? How can I solve that?