I've never used HALion, but I had a similar problem to you just recently, in Kontakt - I was getting clicks when releasing notes when playing a home brew Giga-instrument. The problem was simply that the release times were set very low (1ms = 0.001s) - Gigastudio will not actually reproduce such a fast release, it will limit it to about 7ms. So, in Gigastudio, the releases sounded smooth.
Kontakt, on the other hand, was presumably reproducing the fast releases faithfully, which resulted in the clicking. I bumped the release times up to 20ms
in Gigastudio, and that fixed the problem in Kontakt. Perhaps you have a similar kind of issue - just a guess.