I'll try this out, but in XPS's case it is very much a Heisenbug.

For some people it triggers only when a secondary monitor is attached (that's me, usually - right now I don't think it is on, but then again the fans are going so they may be covering it).

For others, it immediately goes away if you switch off the backlight (didn't work for me).

I don't recall if someone has mentioned this in the XPS coil whine bughunt, but I'll give it a go the next time my laptop starts whining. Thanks!