Hey everyone, I've just purchased a pair of Bose QC20i noise-cancelling earphones from an eBay seller. When I turn the noise-cancelling system on, it vastly reduces the noises in my environment, but I can also hear a hissing sound. Here goes my explanation of what type of hissing noise it is. If you plug a pair of earphones/headphones in to a computer these days, the sound is usually great, with no background noise form the motherboard or fan etc. In the past though, with cheap soundcards, if you earphones/headphones into them, you would get a background hissing sound. This is exactly the same hissing sound as I'm getting with the Bose earphones when I have the noise-cancelling system turned on. Is this normal for them, or are they suffering from an issue? Any advice would be much-appreciated!