I am not for sure, but I found this on the web:
Double Second Time Markers
Other International time and frequency standard stations share 2500, 5000, 10000, and 15000 kHz. Sometimes they can be heard from Salt Lake City when the skip distance is long. One of those stations is BPM in Pucheng, China, which is operated by the Chinese National Time Service Center (NTSC) (Shaanxi Astronomical Observatory, Chinese Academy of Sciences). BPM usually broadcasts Coordinated Universal Time (UTC) like WWV and WWVH do, but during minutes 25 through 29 and 55 through 59 of each hour BPM transmits UT1 instead. UT1 varies up to 0.9 second from UTC and the time difference can be either positive or negative. As a result, BPM’s time blips during those minutes normally lead or lag WWV and WWVH time ticks. BPM uses Chinese female voice identification during the last minute of each hour.
And on the google page I found an explanation that would fit the sound, it says:
"HF Time and Frequency Standard Stations - The S-Meter Sitewww.smeter.net/stations/hf-time-frequency.php
The signal strength is typically about S7 in Salt Lake City during winter nights when ... to hear because of strong interference from U.S. Amateur Radio stations. 4996 kHz, RWM, Unknown, 5550 Miles, Institute of Metrology for Time and Space"
So again I am not realy sure, but it sounds likely to be from another frequency, the time marker on 5000 KHz seems to be strong enough during calm nghts....