I've done some testing on this, with a couple of SB2 15" machines and one 13.5". My experience parallels what others are reporting (google "Surface Book 2 13 coil whine"):
1. On all of the machines that have it (which I believe is most if not all SB2s), the coil whine is loudest when plugged in and the performance slider all the way to the right in "Best Performance." It's also inversely linear to CPU load -- push the CPU to 100% and the coil whine disappears. It also disappears when running on battery. As you've noted, it's not affected by either having something plugged in or being plugged into the base.
2. The 13.5" I tested had absolutely terrible coil whine. It was audible in my home office, which has a desktop that emits a bit of fan noise. So, it was loud enough to be noticeable in a normal environment, and in a quiet room (e.g., my bedroom late at night) it was terribly disruptive.
3. According to what I've read, coil whine is worst on the 13.5" and it's fairly ubiquitous. I think it's unlikely to get a 13.5" without significant coil whine.
4. On the two SB2 15" machines I tested, one had barely audible coil whine in a quiet environment that you really had to listen for to notice. One had coil whine that you could only hear if you put your ear up close to the tablet in a quiet environment. Neither had coil whine that I would consider to be in any way disruptive -- and I'm pretty sensitive to such things.
5. That also accords with what I've read: SB2 15" machines also have coil whine, but it seems to be rare that it's anything close to the 13.5" machines and unlikely to be a problem.
So, I think it's entirely possible that you got a 15" machine that's at the bad end of the spectrum regarding coil whine, and it's entirely possible that you could swap it out and get a better one. Of the two 15" machines I tested, I'm actually keeping the one that had "worse" coil whine, because I just don't notice it that often -- and, I only keep it in "Best Performance" mode when I'm doing something that's pushing the CPU, and that tends to get rid of the coil whine anyways. In the other two performance modes, I'd have to go looking for it.
