Need someone who knows the technical aspects of lenses

OK I sold a Canon 100mm 2.8 macro lens to a guy who tells me that it is rattling too much. From my research these lenses are known to rattle when held upright. I had the lens for maybe a month and hardly used it. I was aware of the rattle but was told that it was normal. Everything about the lens is perfect apart from the fact he thinks it rattles too much.
He tells me that it's an inner barrel stabilizer defect. Does this sound right? I don't know enough about the construction of lenses to know if this is even something that could be an issue. I just want to be sure before I have to send him money for repairs.
I will also contact Canon to see what they say.
Edit: Yes it is an IS lens. Just saw that I didn't mention that. Apparently the 100mm is rattles more than a lot of other lenses.
He tells me that it's an inner barrel stabilizer defect. Does this sound right? I don't know enough about the construction of lenses to know if this is even something that could be an issue. I just want to be sure before I have to send him money for repairs.
I will also contact Canon to see what they say.
Edit: Yes it is an IS lens. Just saw that I didn't mention that. Apparently the 100mm is rattles more than a lot of other lenses.