This thread reminds me of a problem I had several years ago at a different property.....
Very similar problems to the OP, similar effects on tones and SNR, and similar timing theme.
I followed all the usual diagnostic routes (line tests, lighting, heating, phones etc.) for tracking the cause over several weeks, and tried all of the OP’s tests and action including ferrite chokes etc. – nothing. The ISP was helpful but there was nothing wrong with the line (they were correct), even though they could see the effect on the SNR over time. But as Mr Saffron suggests, the only effective and easily accessible way to identify local radio interference is with a radio.
So I followed that route, and yes I could pick up a very weak signal at exactly the same time as the interference around 310kHz and 419kHz. I spent hours tracking around the property looking for the source – nothing. I even resorted to shutting down the entire Electrical supply and every device – the interference persisted (on the handheld radio). So it was nothing internally. The next few weeks were a bit mad, occasionally wandering around with the radio near other properties, suspecting neighbours etc. but nothing…..
I was about to give up and thought I would go for a walk down the fields, radio in pocket, and to my surprise the noise of interference became greater. It peaked right outside a sewerage pumping station, some 1000 meters from my property. Bizarre
Later I looked at the history, time v SNR drops and worked out roughly when the interference was being generated and again visited the area outside the pumping station, but before the interference had started and sat down with a radio and a book. I heard the pumps start, and bingo there was the exact noise!
OK, this could be all a coincidence, so I returned another day and even took my laptop, a 3G dongle and a VPN connection back to the router, and watched again. Sure enough exactly at the time the pumps started, the SNR halved!
OK, so I have a potential source, but how to convince the water company? Actually this was quite easy. After explaining that their equipment ‘could’ be interfering with my equipment, they sent a very helpful and knowledgeable Engineer to my property. I showed him all the research and evidence I had. Fortunately, he was a ‘proper’ Engineer and understood exactly what I was stating. He then suggested we take my Laptop live and visit the station with him. He went straight to the Switch Gear cabinets inside and was actually quite surprised / horrified by what he saw – the installation had been done by a sub-contractor and they had not Earthed any of the cabinets – in effect the whole Switch Gear room was acting as antenna.
The Engineer could only apologise, and set about an immediate on-site repair plan – within a week the problem was solved. They even sent me a thank you letter after the event – a lot of companies in many industries could learn from this example.
So what can we take-away from this?
• The source of the interference may not be local to you, or indeed even near to you, if your cables pass by a source of interference on route to you.
• The interference was affecting other connections for sure, but no-one had done the leg-work to find the source. However, it might be useful to check with neighbours that are ‘tech-savvy’ enough to understand what you are talking about.
• It was surprising how helpful the local sewerage company was when approached directly. Sure there are laws against interference, but most companies these days operate a ‘denial approach’ rather than a helpful approach. The approach shown in this case may have, in due course, saved an Engineers life – remember the entire Switch Gear cabinet was not Earthed!!!
So as Mr Saffron says – a portable radio is a really useful tool in this situation and I hope you find the source of the interference. My connection speeds trebled after the source of the interference was removed.
Oh and the irony of the situation? The sewerage company had an ADSL connection to the plant to monitor its operation, and could not understand why it regularly dropped out – it miraculously recovered after rectification work and never dropped out again!
Hope this post is of some help and if I can help further please do not hesitate to ask. This forum is a brilliant source of information and help!