I have one and it's a great lens. As mentioned there might be oil on the blades which would require taking the lens apart. Not an easy task unless you have the proper tools. Perhaps just exercising the lens manually might free it up.
Well, people flush clean shutters don't they? No, don't go pouring lighter fluid into the helical.
Remove the lens groups & clean the aperture blades with q-tips. Front and rear.
If the oil's in the blade housing it's not going to help, that requires removing the blades & resetting them on reassembly.