Great news!!! I was just noticing this too. I back my bike into the garage right next to an old fridge. The first few times I heard this noise, I paused my backward roll and pressed on the refrigerator door, thinking that it was condensation being sucked in through a bad seal on the door, of course as soon as I pressed on the door the noise stopped, so it must be the fridge. Of course at the time I didn't really think about it being the bike or the fact that I had stopped rolling backwards at the exact same time that I was pushing on the fridge door. Anyway, not the fridge. I had a few people look and listen while rolling backwards, they all heard it too, but had no idea what if could be. I am now on my way to the garage to look for the mysterious sticker.