interesting sounding ultrasound

You have two issues here: Generating the electrical signal, and turning it into sound. To create the signal, you can use any of a number of voltage controlled oscillators (VCOs) and use a low frequency sine wave as the control voltage. That will cause the VCO frequency to slide up and down like a police siren, etc. Note that VCOs tend to have non-sinusoidal output waveforms, typically ramps or square waves with lots of harmonics.

The big problem is converting this to sound. For near-audio frequencies (say, 20 kHz-40 kHz) you can try plastic piezo horn tweeters. Many have strong output in this range, but you will not get a flat frequency response... lots of huge dips and peaks. But its cheap enough to try.

For higher frequencies, transducers tend to be tuned to specific ranges. You could warble a little within a given range, I suppose.

If we knew what you plan to do with this, we might have more-specific suggestions.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis