Buffer playback clicks

Hi all. I'm pretty new to Max/MSP, I'm trying to recall buffers and I'm
using a simple polyphonic instrument to playback buffers on a keyboard.

My problem is that sometimes, seemingly randomly, the buffers pop at the
beginning. I probably need to insert an envelope somewhere, but it's defying
my intelligence. The clicking is very random too, not every time, and I can't
see any sort of pattern or reasoning.

Here is my simple polyphonic code:

Thanks!