When a surface is curved, the curvature causes that part of the object to receive light and reflections from a large portion of the surrounding image. When a surface is flat, it is picking out a very small part of the sky object to base its illumination off of, this is likely what is causing your grain imho.
Seeing as the entire object is the same blurred metal texture though, you can potentially save a helluva lot of render time in this animation. Now this will depend on what else might have to happen in the project, but if it were me…:
Disable GI / indirect illumination, its adding very little to the scene. On that object, 80% of what youre looking at is just reflections on the surface, throw in a pair of large area lights and its going to make very little difference to the visual image but will save you a ton of render time.
Next, open your hdri in photoshop and blur the living crap out of it, and save it as a new version. Instead of using blurred reflections in the render engine (sloooooooooooow) just pre-blur the reflective image and use that as the sky (faaaaaast) Use a second crisp copy of the image as the background in the render.
Thirdly, this model would take post-DOF very nicely. Just add a depth map and blur it in photoshop/AE afterwards.
If you do the first two, your render times will plummet, if you do the third too, your render time will probably be 1/100th of the original, and youll be grain-free.