Hey,
yes agree, exactly like two sticky surfaces taking on and off each other!
Annoyingly it comes and goes. Sometimes its just over the biggest of jolts, other times its near continuous and even the engine starting up will set the noise off with vibration.
It isn't the top piece around the grill, its the lower flat piece with the vent in. I've tried jamming credit cards and folder paper up between the two parts thinking it was an interface noise, but it still persists. The only way of stopping it (and Lexus garage saw this as well) was to place hand on top of the area towards the back.
I'm starting to suspect that its also temperature related. Didn't have the issue in winter, suggesting that its something very slightly moving in a gap.
Chris