I have 2 pairs of noise canceling headphones and both suffer from the same issue of picking up (and amplifying! very loudly) the interference from the GSM radio in my mobile phone. Yes its that buzz-clicking noise you get when you put a GSM mobile phone near a amplified speaker circuit.
And it's definitely the active noise-cancelling circuit causing the problem as turning off the headphones power switch immediately stops the interference noise.
My question is, are there any means to shield the headphones from the phones radio signal? For instance would clamping a balun around the lead to the headphones help?
The only thing I've been able to find searching online is this rather unhelpful forum thread.
UPDATE: Thanks to the answer below giving the right keyword to search on (ferrite) I turned up a similar question, with an interesting point that if the phone is on 3G (WCDMA here in Australia) it should not cause the same noise problem and in fact anecdotally I am only experiencing the issue in poor-coverage areas where the phone is falling back to GSM instead of 3G service.
I'll still bug a ferrite core to clamp onto the lead to see if it helps in the case of a GSM signal.
As an aside, I think this would make for a nice little method for staff on airliners to check for people who have forgotten to put their phones in "flight mode".