are you saying in certain circumstances, when you hold the clutch pedal down, you hear a metal on metal noise? that's probably the throw out bearing. it's a cheap fix, but you need to drop the transmission.
if you are saying you hear a metal on metal noise when your clutch is engaged and you are coasting at low speed, that could be something internal to the transmission. alternatively, you could just have something rubbing on a brake rotor, a loose heat shield rattling around that you can't hear at higher speeds, etc.
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