It turns out I have really decent hearing. So it's unlikely to be tinnitus; at worst I have a sort of hearing after-image when a tonal frequency suddenly cuts out (and now I'm guessing that's an effect of the ear actively filtering - I can't wait to watch that video).
But yeah, I did hear it on the dog. It's so anomalous to hear a high pitched noise coming from an ear that I was willing to question my own hearing/sanity. The best I could manage to guess was that there was some sort of small gas pocket leaking, which made a little bit of sense since my dog (samoyed) had a bubble on his ear (that eventually gave him a sadly adorable floppy ear when it eventually drained - prolly an aural hematoma); the bump never actually seemed to deflate due to the noise, though. At the time I was an early teen just sort of beginning to learn analytical techniques, so that reasoning was the best I could muster. These days I tend to trust/understand my senses more.
Or do you only hear it from dogs?