What is this noise?

Happens when in idle whether gear or neutral at a stop still. Clutch in/clutch out doesn't make a difference.
When taking off I can briefly hear it before the loudness of the exhaust takes over, so I'm not sure if its not doing under acceleration.

Most noticeable when shutting off the engine but is intermittent, sometimes it rattles, sometimes it doesn't.
There's also no noise cruising along

Am I going crazy?