"Paint Can rattle"

So what actually is this "paint can rattle" that I often hear from the L/H side when accelerating a little ?
It sounds like pre-ignition, but know it's not that.

Have you seen this thread? currently under this post
Paint can rattle?Nov. 3, 2015 RKTNMYPKT
Hoopla
 
Nope it's not any of that.
I should add, I'm not concerned about this sound, as I say I know it's not mechanical, was just wondering what it might be.