If you have a female, most likely she won't ever sing no matter how hard you try to train her. There are cases where females do whistle, but it tends to be rare if they do.
She may be getting a bit on edge about the noise so early and is upset by it for some reason. I doubt your whistling is bad. She may just not be used to it and may be in a bad mood early in the morning when you do it. My birds sometimes do it when I whistle too. I think it may also be certain notes that are hit as well.

 

Thats what i was thinking too, that i was hitting certain notes she didnt like because i do a wolf wistle to her everyday and she doesnt have a problem with it but when i start to sing/whistle a full song she gets upset, or at least i think she does!?

 

I have a female that mimicks my males, but not whistling/singing wise, just some noises they do.

 

My mother's female cockatiel whistles but it sounds different than our male, more like a whistle imitation and it's not quite as "sharp" of a sound. I agree that maybe it was notes you were hitting that she didn't like, Laura doesn't like some whistles too.

 

My female cockatiel peeps at me, with different notes, as if she's trying to imitate things around her (TV ads, the simpsons theme song, her name etc) but she doesn't actually whistle- though she is quite noisey at times!
She also makes sushing type sounds (almost grandma like!) when she thinks we're making too much noise!!