Disco 3 (LR3) What's with the knocking??

hoopfish said:

It has blown 2 main beam bulbs since owning it, the connector to the bulb looks like, at some point, it has got hot and gone a bit brittle. I have just tested the volts at the battery and it is giving me a reading of 14.4-14.8 at around 2000rpm.

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What year is it?
If it's a facelift with HB11 bulbs, then it's normal for them to fail in pairs.
Unfortunately the bulbs overheat inside the headlamp casing. Lots of FL2 facelift owners change the bulbs to HID, adding more light and lengthening the life massively.

If it's a pre-facelift with H7 bulbs, then they can also fail in short succession, but life is better than the facelift headlamp.

Voltage is completely normal for a modern vehicle.