Is it ethical to buy noise canceling headphones with research funds?
Disclosure: we actually bought pretty expensive Bose Noise Cancelling headphones for our students recently, for the reasons you mentioned.

I think there is a very easy answer to this - will your funds cover the expenses if the purpose is reported truthfully, or will some "creative" cost reporting be required?

In the first case, I have a hard time seeing an ethical issue - as you say, equipment is equipment, and if whoever funds your research is ok with spending some of it on "potentially productivity-increasing even if ultimately not absolutely necessary" equipment, then why should you be ethically required to not make use of this option? The only case I would see where this would become an issue if you, for instance, would not actually be planning to be using the headphones much at all in the office, and are mainly looking to use them for a private purpose (but then again, if you wrote this in your expense report, whoever funds your research would probably not be ok with this expense anyway, making this more similar to the second case).

In the second case, it is pretty clearly not ethical to hack around whatever the criteria given out by your funding agency (e.g., pretending like you will need noise cancelling for a specific experiment so that it counts as costs for conducting a specific experiment rather than equipment costs, etc.).