You're hearing the gas pistons sliding on the magazine tube. There's nothing wrong with this. When you close the action, the gas pistons will be pushed into the proper place before firing. I wouldn't worry about it.

As for why the new ones on the rack didn't do this, I would think that they were probably assembled without removing some of the packing oil/grease. With the extra oil, the gas pistons are probably sticking inside the flange on the barrel. Again, nothing to worry about.