December 2010: This particular noise on a bearing of 265 degrees has been going on for at least 16 months. You can hear the interference as well.
In the late autumn I spent some time trying to track it down without success. As can be seen in the spectrogram to the right, the interference slowly drifts around. It sounds like a very large industrial electric motor starting and stopping. Because of this, I thought it was originating from one of the many industrial estates to my east. So I drove over to the other side of town but could not find anything. In the end I left it as I had worse noises and I was quite busy work wise.
More recently I have found the noise to be more annoying as it sits in the same direction as moonset and prevented me from hearing any EME signals coming in from around 270 degrees.
After a re-evaluation, I decided during the Christmas holidays that I could not find the noise previously because it was, in reality, originating from a domestic property much closer to me. So I plotted a line at 265 degrees from my house (which can be done automatically on Google Earth) and saw that it went through four house just up the road from me. This coincided with a visual alignment taken from antennas when they were pointing at the noise source. Later in the evening I walked up the road with my HB9CV and found the specific house where the interference originated from - it was the one I thought it was I'm pleased to say.
It's such a weird sound that I was still unable to think what could be the source. However, over the last couple of days I found out that it comes on at 17:00 and goes off at 22:00 in the evening and from 06:00ish in the morning and going off at 10:00. This provides a clue as what could be the source of the interference - a central heating pump!
I am now convinced it's a central heating water pump by looking at the nature of the image above.
The noise disappeared exactly at 10:00 when the timer would automatically turn the central heating off but it came back within one minute when I suspect it was turned back manually on as it is a holiday. It can also be seen that there was quite a dramatic change in frequency as the water started to circulate again.
Well, I could be wrong but I bet I'm not!
It looks like I need to go knocking on a door again!
Convinced it was a central heating pump I walked up the road and knocked on the door. I slowly explained what I thought the problem to be - brushes going in the motor and that his central heating went off at 10:00 and was immediately turned back on again. He was quite intrigued and agreed to come back to my shack to have a listen to the noise. While there, he phoned home and asked his wife to turn the central heating off and back on a again which resulted in the image shown on the right. This confirmed that I had identified the right house and the exact cause of the problem.
We went back to house with my wire loop and FT-847 and went to the pump located in cupboard. I called on my wife on my mobile phone while wiggling the pump. While I was doing this the plot went mad finally confirming the problem was indeed the pump.
The owner very kindly agreed to get a new pump after explaining that I would expect it to fail within a reasonable time.
Another interference issue solved I'm pleased to say.