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Last night, a rather strange noise woke me up around 5 am. It was like if a normal thunder sound had been fed to a reverb/delay box. As I was getting back to sleep afterwards, I could hear the sound of rain.

As I have been producing acid house tracks the whole day (it's just a hobby), using quite a lot of delays here and there, i'm rather skeptical of myself. Yet it seems I was woken up by a reverbed thunder sound (the reverbed part of the sound didn,t last longer than 5 seconds).

As I live under high-tension lines, I'm wondering : if lightning falls on them, can it produce the kind of resonating metallic sound I heard last night ?

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I can't tell what exactly you may have heard, but sounds reflect off of all kinds of things, particularly flat walls, so echos from a loud sound like thunder is quite plausible. 5 seconds is about 1 mile of total propagation, so that again is plausible.

Lightning is a large current that ionizes the air in its immediate vicinity. Such ionized air is conductive. Having more conductive air near power lines can cause arcing between adjacent conductors in extreme cases. It can also cause more than usual corona discharges as the remnants of the ionized air waft near the high voltage conductors. This is most likely what you heard. Under the right conditions with the right imagination, I suppose this could sound "metallic".

Stand under the power lines, particularly on a humid day, and you should be able to hear some crackling. I suspect you heard this effect cranked up due to the wet air from the rain, and the short lived remnants of the ionized air caused by the nearby lightning.

Somewhat aside, note that lightning doesn't actually hit power lines directly. Look carefully at the power line you mention, and you will probably see three main conductors. These will be the ones specially insulated at the mount points of the towers. Then there should be one or two cables above those that are not insulated and will be directly connected to the towers if they are metal. For wooden poles, they will be deliberately connected to ground via a thick cable every few poles. These upper lines are there to catch lightning so that it doesn't get to the actual power lines.

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