Drink machine? I'm trying to think of what else would cause that mid-range constant...
In RX, use the vertical selector, zoom in, and select just slightly more than the hum itself, and then just do gain reduction. It's sooo much simpler than Audacity in that regard.
As for figuring out where it is, there is a isolation method in Izotope's products (it varys product to product). In RX, select a 200hz band (using the box selector, not the vertical or horizontal ones) and make sure it's plenty long so your audio doesn't stop while sampling. A couple minutes worth should be fine. Now as the audio is playing, you can move that around and hear just certain frequencies. Now hit the preview play and move it around until you clearly hear the hum and that should give you the vicinity. After that, it's just as has been previously described.