Metallic knock/rattle on e60

Recently I've been conscious of an intermittent metallic knock/rattle on the e60. It seems to be coming from, the upper half of the car but at times I think its coming from the vicinity of the headlining/windscreen and other times I reckon its coming from the bulkhead/engine/underbonnet area. It seems to happen on the deflection phase over bumps, but not consistently.

I've check all the obvious stuff for looseness - the headlining is firmly in place, there is no sign of energy/vibration when the knocking occurs, the mirror is solidly in place, there is no sunroof, the wiper arms, etc. don't have play and aren't tapping off the bonnet, the bonnet is tightly closed and no obvious loose fittings anywhere and none of the major components in the engine bay seem to be loose.

There's no obvious signs of any issue with handling, steering, shock absorbtion, etc. so I'm pretty much ruling out bushes, balljoints, wishbones, ARB bushes, etc.

Anyone got any ideas? Its annoying me a bit like Chinese Water Torture might!

All replies appreciated!

Quite likely to be your strut brace, mine did the very same a number of years ago. It was tightened and a bit of grease applied and never occurred again !!