There is a remote possibility that the voice coil of the woofer on that speaker is slightly burnt. Maybe one coil of wire at the end of excursion is off the coil former, for example.
Running the sweep may have heated the coil when the lower frequencies were run just enough to get the aberration at 180 Hz.
Running the sweep a second time may have happened after the VC had time to cool.
As I said it's an extremely remote possibility (because it did not show up at other low frequencies). You will know, because it (or "something") will come back if that's the case; voice coil damage is cumulative. So run them as you normally would, and sooner or later it will either come back, possibly get worse, or if nothing is wrong, you won't hear anything unusual again.
You also might want to pop the covers (if they're not off now) and have a close look at the woofer dust cap. Sometimes the glue can degrade over time and it will become loose or offset. It's easily repairable if that's the case. I've seen it happen (once). It's a simple repair (re-glue it). It caused a noise at higher sound pressure levels on certain program material that sounded like a burnt voice coil, but wasn't.
It's also a little curious because 180 Hz is a harmonic of North American 60Hz AC frequency, so there is almost always some noise at that frequency, and although it should be well down in level, it's typically measurable. So it could suggest an issue with the power supply filtering (anytime you have capacitors, age or failure is a possibility). Again, another remote possibility, and one that will come back sooner or later if that's the case.
Finally, especially with any digital file, you can never rule out a momentary failure to properly play the file, in that case it would simply be coincidental that it happened at 180Hz.
