This plot clearly shows an approaching lightning storm.

The gradual rise in Noise Margin as the sun rises. It's accompanied by a similar fall at sunset. It's often a lot more pronounced than this - especially in bad weather and the winter.

The effect of a pulse of electrical interference. As the Noise Margin is reduced after the initial pulse, it's possible that this was caused by an electrical item switching on (and staying on).

Compare with the lightning plot in which there is no step-change after the pulse.

The electrical item is not necessarily local but it is probably somewhere on the route my phone line takes back to the exchange.