What is This Wide Band of Noise on 40 Meter?

Yesterday I felt lucky to have pinpointed the exact source of two narrow bands of radiofrequency interference noted on 20 meter band as coming from a small SMPS powering an amplispeaker of the computer. Today morning another wide band of RFI has popped up on 40 meter amateur radio band, which I have not seen earlier.

40 m band seems to be open for CW DX as I can hear some stations at high speed at the very low end of the band. It was while I was trying to relearn copying CW that this band of white noise of about 15 kHz bandwidth popped up early in the morning. Audio is like a motor boating noise. While other signals on the band were fluctuating and could be from digital radio and CW, this one is steady. I checked the three device power supplies connected to the UPS, by switching them off one by one. None of them are the culprit in this case. Now the saga of finding the source of RFI starts once again, though on a different band. As you can see the strength of the noise is reaching almost 20 dB over S9! It was not there when I switched on the radio. Currently it has been there for nearly 30 minutes and is still continuing. Being early morning, there is no appliance running in the kitchen or anywhere else at home.

As I was typing the last sentence in the blog post, it has disappeared! But I observed, another fainter band of different texture appearing transiently higher up on the band, which disappeared soon and came back intermittently. The initial wide band at the lower end of 40 m band also appears intermittently, though for longer periods.