Welcome to the forums, @arenivas!
Does noise stand out to you anywhere else in the photo? If the excess noise is localized to the area of focus, then I think the “graininess” has more to do with the texture of the subject than a problem with your camera. Thankfully, from what I can tell from this shot, I would say your camera is working properly! Also, I do not know of any way that your lens per se actually ever increases/decreases the effect of noise in a photo.
You mentioned this was a 4-shot panorama. Was that to get the effect of a wide-angle lens, despite being limited to a 50mm? Gotta say, that’s pretty creative!
Was the stitching done in-camera? If so, that’s where I’d look first to the possibility of the introduction of unwanted noise.
As for the critique of this image, I think the shallow depth of field is a good choice, considering the amount of leaves on the path, the relative lack of leaves on the trees, and the lack of an emotive far-distant subject. I would tend to say there is an imbalance of pure white sky to the amount of path that is readily detected as “path”. As suggested before, I may have tried focusing on one, single, main-subject leaf (or arrangement of leaves) closer in and maybe from a little higher, so there would be more path with less sky.
So that’s my two cents! Autumn is my favorite season for photography. 🙂