Strange movements in animations.

I’ve really been messing my head on this one, but to no avail.

I have an FPS arms rig and a skeletal mesh which I already connected to the rig, but when I got to the animation part I noticed that there’s some strange slight side to side movement which isn’t supposed to be there. And technically the arms clip and move through the skeletal mesh I connected to the rig.
I’m not sure what’s causing this? Is the rig broken? Are the IK’s causing this? It seems absolutely fine in Blender but when imported into UE it breaks. I’m using UE5. It would be great to get an answer, thanks! If needed more info feel free to ask.