mystery frequency

I am picking up in the DC area a few tones on the frequency 153.635, It seems to happen every 3-5 minutes, sometimes higher pitch sometimes lower. If anyone knows what this is I would be happy to know. Thanks!

I have stumbled across a frequency that is pretty strong in Cloverdale, Northern Sonoma County, San Francisco Bay, California. It is 451.89375Mhz and the FMP prompt shows that it is an NXDN48 system as seen by the copied line below. I have tried searching for System 479/L-479 in the RRDB with no...