On my guinea pig mandolin (the $25 one I'm going to completely take apart and rebuild just for grins) I noticed that the neck has a dip in it, maybe about fret 9 or 12. I don't know if it has a truss rod--never thought to look. I'm not talking about a bend, but an actual dip in an otherwise straight neck. I didn't take a photo, but it looks kinda-sorta like this from a side view: ----u--- ... not that severe, obviously. Is this something a truss rod adjustment would fix if it has a truss rod? Or is it a re-plane the neck or replace-the-neck kind of thing?