Broken Hallelujah

I have a question for anyone who may be able to help me out. My PC tower is humming. Not the soft whirring sound, but humming a little loudly. And not as soon as it's turned on, but after a couple of minutes, it starts loudly humming. It's 3 years and 10 months old and I've not heard this sound until lately. It can't be good news. It's been happening for about two weeks now and usually I can power off, wait 20 minutes, and turn it back on without the humming. Then it'll hum again an hour later. I've cleared caches, I've defragged, I've even taken off the cover of the unit and dusted everything with a makeup brush. I've checked all the connections, reset it, and looked in Windows "Help" area.



I've been telling myself if I'm not getting any errors about it, it's all right. You know how you usually get warnings about every little thing? Has anyone else had this experience before? Any advice besides to take it in and have it looked at? Keep surfing until she blows? Anyone with a tower for sale, like really cheap?