I found this in the firmware notes. Is it relevant? Maybe not given the date. Though the noise I’m hearing does appear to be caused every time the 218 begins feeding the speakers a new signal.
Originally Posted by DSP3200 Product Firmware Release 1.0.3 (2011-07-07)
Software Fixes - Critical - Software must be updated
Improvement to protection strategy for dealing with interruptions to incoming digital streams.

Finally the proud owner of a 218 and 3200's... Will they grow up to be an 818v3 and 8000 se's???
Isotek Sigma Evo 3 power conditioner