an update on this - today I recorded the output from my test patch. You can download the audio file and the Pd patch here.
This gave me a few clues. The first was that the output gain was very very low - it barely registered on the meters of my sound card when using line-in.
Here’s a spectrogram of the (normalised) sound file.
I guess what I’m hearing at the buzzing noise is those very clear lines going all the way up the spectrum!
This therefore feels to me like I’m simply hearing the digital noise floor, because for some reason the output gain from my Seed is very high.
This also may relate to another issue I’ve had, where my Pd patches need their output gain reduced much more than I would expect before they can sound good running off the Daisy.
Perhaps I’m completely misunderstanding how the output gain stage works on Seed, or pd2dsy?