So my question: could this knock somehow be related to running low coolant for a short amount of time, even though my car did not overheat?
how old is the timing belt?
and the tensioner?
if it is rod knock,
unplugging the plug wire should make it go away, or at least greatly reduce it.
a mechanics stethoscope will help you isolate the noise location.
rod knock should be down the center line,
tensioner noise will be up front on the driver side.