If I "push it" on the treadmill or during a race, for a sustained amount of time (as little as, say, 10 minutes), I will likely experience something that is a little hard to describe but I will attempt to do so. It is basically a metallic "taste", although it seems to be originating from my head, and it's more like a general sense of metal rather than a taste. If my brain had tastebuds, this would be what I'm experiencing. This may sound ridiculous to you or you may say "that's it!", I suppose your mileage may vary (pun intended).
I thought it might be blood, which would account for the metal sense, but I never see any blood.
I am taking no drugs and do not have any diagnosed medical conditions, and I'm 40 years old. I think this has been happening ever since I had the ability to run, so pretty much since tykehood.
Here is an interesting link but I'm still not satisfied. The mention of "taste phantoms" seems like a possible jumping-off point, though.