I’m a super-new whistle player, and I’m already in love with it. I’m practicing several times a day because I simply can’t help myself, and I thought I was making pretty decent progress. My cats, however, seem to disagree. I really have no way of knowing if I’m in tune or not, because all my playing is masked by three cats howling.
I’m starting to develop a complex–in addition to overly critical cats, my husband bought himself a set of noise-canceling headphones recently.
Sheesh.
-Savannah 
I’ve learned putting my headphones on while I play actually helps me hear myself better - AND i’m not flinching away from those really high notes. 
I had a corgi that would howl when he first heard me play the whistle… he got used to it and started laying at my feet when I played instead. My girlfriend has a coon cat that tries to jump into my lap when I play at her house. That cat really seems to like it. The other cat is indifferent to it. Since you are new to the whistle, you need to remember that you brought someting new into their house. So give them a little time to adjust, they’ll be fine, and so will you.
you brought someting new into their house
.. and this can be true even if it’s not “their” house 
My neighbour’s cat often visits - she’s quite happy to sit and doze whilst I tootle around on any of my homemade plastic whistles - as soon as I get out a “commercial” metal whistle she’s off. I’m guessing the metal pipe has more high-frequency response than the plastic, something she may not like. You might try muting the whistle slightly, drape a handkerchief over it maybe, to try reduce the high frequencies, else there’s various solutions involving bits of card and/or blue-tack (poster putty) etc which help reduce the volume of the whistle … not to say it doesn’t affect the tone of the instrument, but it may give you a more agreeable practice environment 
Don’t take it too personally. I’ve had MANY cats over the years and they tolerate the whistle at best, more often dislike it and leave the room when I play. I’m thinking the high frequencies are not pleasant for them. That’s my story and I’m sticking with it.
Is it hatred, or an attempt to join in? ;-?
Cats are odd critters - some love whistle, some loathe it with a passion. Ditto for other instruments.
Our current cat loves the whistle; if she’s out in the yard I can call her in by playing. If I play sitting she’ll try to climb into my lap and rub her head against the whistle, purring ecstatically. But she’s terrified of the Uilleann pipes, as several board members can attest. If we have a house session, she’s underfoot until/unless the pipes come out, but after the first faint drone she vanishes for the night.
My cat just leaves the room when I play. Everybody is a critic.
I bet!!! Have you checked the neighborhood after playing to see if any other animals have remained by choice?
I had a classmate who was banned from bringing in his pipes after we tried to play them for our production of Brigadoon.
My cat doesn’t seem to notice, but my boxer sits in front of me and howls. Since he’s not running away while howling he must like it.
My cat did the same when I first picked up the whistle. Now he sits and sleeps in the same room as me quite comfortably. The same happened when I started to play the guitar and I suspect the same will happen again when I try my hand at the melodeon.
Our dogs completely ignore the whistles (even the painfully high G), but one of them starts singing whenever Mr Husband plays high notes on the harmonica…
Let’s say I’m working on a new tune and I’m playing it well enough and the tune’s natural energy is coming through: if my cat likes the new tune she’ll race around like mad, jump into the easy chair and do crazy somersaults, occasionally stopping to peer wildly at me to see if I’m paying attention. It’s hard to play flute and laugh at the same time. Which is probably very much the point (“So you think you’re a musician? Take THAT. We’ll see how rock-steady you are.”).
If I’m playing in lackluster fashion and/or with fumbles, she’ll just lounge there looking at me with a heavy-lidded sardonic gaze that definitely reads, “Seriously?”. She’s actually rolled her eyes at me once or twice. When even your cat is so unimpressed that it lets you know: man…that hurts.
The dog that we had when I was learning the whistle never enjoyed the whistle, even when I didn’t make squeaks and squawks anymore. Our dog now likes music. A friend’s dog would sit on my feet when I played my flute. Wouldn’t even move when I tapped my feet.
The most musically aware dog I’ve met was my piano teacher’s golden retriever. Whenever I struggled with a particularly difficult passage, he would bring over his cuddly toy and press it firmly against my kidney area. It didn’t help my playing much.
Awww…how sweet is that? 
We have two Border Terriers that seem to like my playing 
They usually sit next to me once I been playing for 5mins or so.
Probably just bored with their bones 
Our cats are pretty much indifferent to anything I play. I am more likely to meet opposition from my wife and daughter. 
The cats are mostly interested in “persuading” me to give them my attention 
Neither of our cats is impressed with my whistle or recorder playing - both leave the room when I pick up an instrument. I’ve told this next story in another thread but it bears telling again: I was busking one evening at a downtown event when a couple walked up with a small collie. They listened for a bit then I noticed the dog was moving her mouth and prancing around like she was trying to sing and dance to the music. The couple said they’d never seen the dog do that before. Guess I missed a chance to have a really cute singing dog act.
walrii, I think this guy might be available. And you get an accompanist to boot.
https://forums.chiffandfipple.com/t/a-night-at-the-pub/74769/1
My cat leaves the room when I play my violin. But then again…everyone leaves the room when I play my violin.