I feel your pain (actually, your heat.) Same problem in my office, and it gets worse in the cooler months when the building A/C shuts off. Our solution ended up being one of the freestanding A/C units in my office, which cools the room just fine. The problem is your use of the word "quiet". Between that unit and a couple of extra fans to circulate the air, there's a pretty high level of "white noise".
If your boss/company owner/whoever is paying the bills will understand this and work with you, see if you can find a unit you can mount above the drop ceiling and then duct it into your office. But plan for the worst-case scenario of a water leak and make sure to NOT mount it directly above any equipment, tape storage, etc. It happens (trust me on this one.) I had this setup at a previous job, and there was not the noise level I have now, nor did I have to empty the condensation bucket every day. It will cost more money, but IMHO it's dollars well spent.