Allrigth so putting in a little spacer (1.5mm metal) under the piezo and over the first part of foam, i had to put gain down to 4-5. but still same results. Seems to "burp" when making a snare roll.
Just use a 1-2 mm foam disc (EVA, double sided foam tape or similar) instead of something made of metal under the piezo, and get rid of the other foam under it.