The day after I got my car, an annoying rattle started in the cabin and I suspected the roof. It really did start to drive me mad and I didn't want to post on here to put anyone else off.
Initially I checked just about everywhere to try and find out where the rattle was coming from. I wondered if it wasn't the roof and in fact checked the glovebox, sun visors, passenger seat and headrest, door storage compartment as well as the passenger door! On one occasion I even half folded the roof down to see if there was anything slack or loose on the folding bits of the roof. Eventually I discovered that it was the inside trim at the rear glass window - there's a point where I think 3 different bits of trim meet right back at the glass area. As soon as I slightly pressed on them the rattling stopped, but there definitely was some slack that was making the noise.
Eventually I might get a bit of velcro or something to tuck in behind the pieces of trim just to hold them together, but the cabin is now blissfully rattle-free again.
Reading some of the issues with the E93 roof, I was starting to get a bit worried, but I think since the Z4 is only a 2 piece design, there should be less rattles and noises.