There are two things on external PSU, they only have one rail, so they are more easy to repair, and if that's not possible you only need to find a PSU to fit that rail, not 5 different as in many internal PSU. Of course the monitor will have DC-DC converters inside, but they are usually more reliable, not facing transient from a power transformer being unplugged next to it or things like that.
I thought is more common every day to have external PSU for monitors and even TVs, I always try to make that choice, it's annoying to have the external PSU instead of an interlock or 8 wire straight to the monitor but I think it pays off. I don't really know about it as a noise source but just an idea. I have an LG monitor I bought one year ago, external PSU, still I don't really like it and I think next time I'll go for a different brand I do like the external PSU.
Good luck with that and I hope you find the solution. I've seen topics around about gammers complaining about whinny graphic cards, solved with some glue on the inductors.
JS