There are two territory holding male Goldcrests in my local park. Each appears centred on a large cypress. This morning, over the traffic noise, I head their incredibly high pitched, not very loud song, which appears to come from a tiny violin.
I searched for it a bit halfheartedly, as this cypress is massive, but the Goldcrest just moved onto a smaller one. It fed, as Goldcrest do, in a rush, never stopping, but it did stop for a moment to utter his song and I managed to capture it.
This is a sonogram from the wonderful website Xenocanto of a Goldcrest song: