Consider the obvious. The tweeter is a transducer, it has no intrinsic energy of its own (other than the kinematic energy imparted to it by gravity which will not produce a sound unless you drop it on the ground). If you take the tweeter out of the speaker and hold it to your ear you may hear noise. Obviously the tweeter alone has no power going to it so it can't be 'transducing" so what you hear is similar to what you hear when put a seashell to you your ear at the beach (go look that up, the education will be good for you).
Tendinitis is another possibility for a un-powered tweeter appearing to be hissing. If you have a mild case of it there is there is a chance that by concentrating on the tweeter, you are perceiving the results of this medical issue.
Otherwise this question is wrong. One must as 'Where the signal coming from?" The answer would have to be the electronics driving the driver. Something is producing energy, this is a great trouble shooting exercise to sharpen you inductive (not deductive, no matter what you think of you readings of Sherlock Holmes, he was a master of inductive rather than the deductive reasoning processes. Again, do some homework, it will be good for you.