Can you render a section of a UST for us to listen to? That example is so short it's hard to give advice. If it was happening with just that one voicebank I'd say to check the oto file and make sure there's not a pop at the start of the first note, but if it's happening to every voicebank and resampler I think we need a longer demo to really get to the root of the problem.
Try clearing the STP in the note properties box. Highlight notes, hit CTRL and E to get to it.