Most mornings I have S9 level noise on 10m.  I will last for 10-20 minutes and then  go away (only to come back later).  Today I noticed that there was also a 2-3 kHz wide (that's the width of my SSB filter) noise signal every 18 Khz or so. The S9 noise was much weaker (S4) on 15m and the 18 kHz signals were there but very weak. Has anyone else seen this?  Any idea as to a possible source?