Just got a new U12S cooler and in the bag of 'common parts' is a clear silicone strip about 3" long with self adhesive tape on it.
It is not mentioned in the instructions.
For anyone interested, I just heard back from Noctua - here is their answer.
"It is the predecessor of the Anti Vibration Pads, so it also helps to reduce vibrations. However, since you have the Anti Vibration Pads, you do not need the strips. "
At this point Noctua have 3 variants of the U12S, the vanilla(beige and brown), the Chromax and the Redux, which one do you have? Could you include images of what you're seeing? Considering it's not mentioned in the manual, I'll need some time to find it off of Google. host your images on Imgur or their ilk and then parse them here. Perhaps this?
However......the web site refers to "antivibration pads to offset a second fan by 5 mm to improve acoustics when using a second fan"....maybe cut that strip into appropriate lengths?
For anyone interested, I just heard back from Noctua - here is their answer.
"It is the predecessor of the Anti Vibration Pads, so it also helps to reduce vibrations. However, since you have the Anti Vibration Pads, you do not need the strips. "