my cats usually did not like the guitar too much, although my current kitty doesn't seem to mind.
We had one cat who hated, I mean despised the sound of a casio keyboard we had. One day, I was in the basement with my guitar gear, and my brother turned on the casio and played a few keys. Within seconds, Sampson, our huge strong black and white male cat came dashing down the cellar stairs. Leaping up on the counter where the keyboard was, he jumped on the machine and slapped at my brother's hand, in a non-friendly fashion. The message was quite clear. DO. NOT. PLAY. THAT. INFERNAL. THING. EVER.
He did this on another occasion, and it was witnessed by my sister. he didn't like that keyboard, no sirree.