oil lamp///

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Hello everybody a bit of help or advice needed please, i have a 2002 terios had it for years with very few problems but today started it up drove about a 100 yards and the oil lamp came on and has stayed on ever since. the oil level is fine but i think when i rev it it may sound a little rattly on top, but i am not sure. could it be oil pump or faulty oil switch?
Any hints or tips appreciated, any one had similar problem? many thanks.