Honda J Series Engine Knocking Timing Belt Tensioner 

If you Honda is making a strange rattling or knocking sound that goes away once it's warmed up. It more than likely has a leaking timing belt tensioner. On cold mornings it can also set off a CEL for knock sensor for all cylinders.
It's best to replace it sooner than later or face catastrophic engine damage when your timing belt slips a few teeth.
Aisin part number B00829H1JO