Metal to metal?
When my lower arms were completely shot, I could accelerate on the motorway and the car would veer to the central reservation, and when I come off the accelerator the car would pull to the left. It was a tricky job keeping it in a straight line. But even that bad it didn't sound like metal to metal.
Get yourself under your car and have a scout about. Check the bushes on the lower arms to see if they've split, or even disintergrated. Check you anti-roll bar to make sure the bushes on these are ok aswell!
Also, jack the car up and do the test to see if a wheel bearing has gone. Do you get any whining noises to accompany the knocking?
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Quote Clarkson, Top Gear, 17th July: "I'm not having a stroke... just driving a Nissan!"
Quote Ry, Mallory Park, 3rd Dec:
"Ahhhh crap... Just a bit too over eager really!""I used to be an adventurer like you, but then I took an arrow in the knee"