I too am having this issue with a single H9 in PRE/POST mode. it happens when i switch between a preset in PRE and a preset in POST, and the inverse too. It also happens when i change a preset from PRE to POST and inversely. Its louder than the instrument signal itself.  H9 is wired in front of (PRE) and in the effects loop (POST) of a Fender Bassbreaker.