I only ever used the A20 once or twice, but I have loads of hours using ANC headsets (Softcomm and D&C ones). Mine do have a constant bit of white noise at a relatively high frequency when switched on, but it should not be more than a running tap. It depends a bit on the quality of the headset and of the state of your hearing whether it is noticeable or not. At a guess, the A20 should not produce a hiss that loud.
Consumer style ANC/ANR headphones are better at this, but aviation headsets may use a more aggressive noise cancelling approach and this may (could) lead to this effect being more noticeable.