I don't know if anyone else has had this problem with Sampletank 2 and Tracktion when rendering into the project directory on a mac; sometimes the start of the resulting file sounds really 'phasey' or something and then, after a bar or so, sounds as it should. This seems to happen particularly with string/pad sounds playing chords I have noticed. The workaround I discovered is to extend the start of the midi clip a few bars and place a single note at the start with it's volume turned down to zero. This seems to trigger Sampletank and then, by the time it gets to the part you want to hear, renders correctly.
Hopefully this helps someone
Steve