Burning propane smell from 2 vent-free heaters hooked to the same tank?

We have 2 propane gas log heaters, both hooked to a large tank outside. We have had these heaters for a few years and both have been cleaned and even one just had the whole pilot mechanism replaced. For some reason, when the 2 fireplaces are turned on, we get a burning propane-like smell. We do NOT get that smell unless either of the fireplaces are on and burning. There is no soot coming out of either. There are no propane smells coming from outside near the tank. They just came out to fill the tank, since I thought it may have just been empty. What could be causing this smell? I don't ever remember this smell in the previous years. The fireplaces have been running for a few weeks, so, it would seem that it would have burned out anything that has aculated over the summer.