I can't say for certain, but I think I know the issue and you may be able to solve it with the airline adapter from Bose.
So your QC20, if you look at the jack has 3 bands (Left,Right,Mic/remote). If you look at the extension it's likely to have just 2 bands (Left,Right). Si I think the "noise" is being created by incomplete contacts and the mic/remote band interfeering. This is common on aircraft when they only have the one jack input (rather than the 2 prong input most used to use). So the Bose Airline adapter can be used in one or two prong "mode" and I suggest using this between the Bose and the extension to remove the noise.
I have seen this improve sound before when people plug headphones directly into amps for the same reason, so it's a cheap solution to try