I bought the new XM4  headphones to replace a pair of Bose QC 35 but the noise cancelling on full produces a background hiss.

This isn’t the hiss in one ear cup that some other people have referred to as a fault, this is general background hiss across both ears, very low level but audible enough to be annoying. 

the QC 35s didnt do this and i tried a pair of Bose 700s to compare and they didn't produce this either. Did i just get a faulty pair or this a function of the NC on the Sonys?

Thanks