Burning Rubber

I did a training run Monday morning on the treadmill at home.  Our treadmill has been giving us trouble for a while.  It's fine as long as you don't try to adjust the incline.  It's stuck on a permanent 2% - which I think is actually 0 or 1.  If you mess with the incline button at all, it comes to an emergency stop.  You have to shut the power off and turn it back on in order to get out the error mode.  I was doing 1/2 mile walking, 1/2 mile running intervals.  A little past the 3.5 mile mark, I started noticing a burning rubber smell.  I checked my pace to make sure I wasn't shuffling, which can tend to cause that friction smell on some treadmill belts.  I carefully looked all around to make sure nothing was touching the belt. I couldn't see anyting but being the fool that I am, I didn't actually stop running and get off the treadmill and look all around it.  I decided to keep on running.  The smell stopped during the next walking interval.  I didn't notice it again when I started the next 1/2 mile.  Perhaps I was in such a zone, that I was literally burning rubber?  Or is it more likely, it's past time to start looking at a new treadmill?