I've used/worked with coreboot(ed) thinkpads quite a bit over the years.  I believe what you are describing is known as the, 'coil whine...'

...and if I'm not mistaken, it has to do with power delivery.  In my experience, in the beginning, I often observed that my Thinkpad x60 would make a terrible whine that seemed as if I could audibly hear the CPU's calculations, heh.  However, if I inserted a USB stick, it would become much more acceptably silent.

I believe it's just a quirk of the boards, and has to do with power delivery to various components, and perhaps most specifically, USB.

The tried and true method of trying to mitigate this, is to mess around with, 'power top,' and attempt disable any power-saving settings that are automatically enabled by default - USB being a good place to start.

For example, if you have ever used a dimmer switch, for indoor lighting, you might notice a similar phenomenon: a slight audible whine coming from the dimmer circuit.  My electrical knowledge isn't up to snuff, but, I believe that same sort of thing is what is going on.

So, yeah.  @zapper you'll want to research and look into 'powertop,' and see what that is about.  And as a quick test, you can just insert a USB stick, and see if that has any immediate change on the coil whine.

The rest, is beyond me; that's about all I know.  On some Thinkpads it's a bit worse than others, but, they all seem to to emit some sort of noise like that, by default, under specific situations.