It looks like m_coef can change suddenly from one buffer call to the next.  If m_coef changes, one buffer will be processed at one value, and the next buffer will be processed by the new value.  This causes clicks and pops in playback.  We have made changes to the engine recently so that changes in processing parameters are gradually incremented from the old value to the new value. See enginefilterblock.cpp for an example.  Please do the same sort of ramping for m_coef.