Do you have any cats?

Or could have one of the cats gotten caught up in the treadmill?
Sorry!

Im wondering if it is being caused by the rubber track being out of alignment enough to be getting hot and almost burning, hot enough to make the rubber smell. You might also consider it to be an electrical issue where wiring is getting hot. I would call the mfgr and get their take on it. Im sure the warranty is long up being 6 years old, but they should still offer advise.

I assume it's a hot component but if it stinks when it's off too, I'd be having someone dismantle it and look for dead mice etc. That's my pick.

Thank you for the replies.
Lucy, I did have one cat at the time the treadmill started smelling bad. No animals in the house now. The treadmill was in the same room as the litter box. I did look but didn't SEE any indication that the cat had done anything rude to the treadmill :o Just in case tho, I've washed (more than once) the walking track and sprayed it with Odoban several times. It has a faint smell but the main culprit seems to be the grease on the rollers.
sierraeast, I contacted Nordic Track when the problem started. They sent a rubber gasket or something along those lines for me to replace but that wasn't the problem.
pjb999, it's not a "dead animal" smell but thanks for the suggestion.
Monica