When a radio is tuned for ONE Specific frequency, all the effort goes into that one frequency...
You said you tuned to 1000hz
So any sort of audio CLOSE to that freqoency of tone, will also pass thru easier than the LOWER tones or, in reality, any range of tones above and below that "tone tune" will not pass as well.
The simple Cobra 29 is set up to pass more voice than whistle, not saying it can't be done, your radio proves this, but the tone you're trying to pass thru is not your true voice range.
Many people would open up the radios' band pass to make it more Hi-Fi - Wi-Fi on 11 meters,.
But the effort of using the whistle for a swing in a tune up of 1,000Hz tone is pushing the upper limits of the radios own BUILT-IN band pass...
Besides, you have a Big Huge CHOKE called a Modulator you gotta push all the colors thru...tends to kinda limit the whole effort of making it pass - when you tune for one side of the spectrum and forget there's' a whole bunch more that makes up your voice - and all for a silly little meter called a BIRD that you feel is more important to watch swing, than to pay attention to the actual conversation and saying "Hello".
The idea of the Modulator is to change your Audio voice component into a range of frequency as a power to be mixed in with 27MHz and call it AM.
The choices of Capacitor value used (say 330uF to as much as 1,000uF) and the Modulators' number of winds themselves, limit your ability to pass everything - it's "pre-set" for a range of vocal tones, not just whistle.
So you're limited right from the start, and why some people use Galaxy's AM Regulation method to pass more audio - also at a price - so it's a give and take.
Please, don't focus so much on Bird watts, they get you into trouble every time - focus more on sounding good to others, that's where the money is, or at least you should throw your $$$ it in that direction - people hearing a good sounding radio makes your voice, your thoughts, get heard, and understood, not a whistle.
.
