I built some small wood shelves for the rear corners of my BH TT. They will be used just to place a phone/glasses etc on but Im courious when i go to secure them into wall whats actually in there. when i tap on the wall (both sides in the corner) sounds like sheetmetal of sorts with a echo-e sound. will i need anchors? I will predrill but sounds like metal, this correct? oh, when im done, i will post up some pics in the mod forum. thanks!