Are you sure the whining is coming from the engine and not from the stereo speakers?
Interference from the alternator can cause a whining noise through the speakers when the radio is on.
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You can check this by, erm, turning the radio off.
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Usually wise during a test drive anyway.
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Does the whine alter when you rev the engine up or when the actual road-speed increases?
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If it alters with engine it could be belts or the alternator or a/c pump
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(assuming it's not actually a knackered engine).
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If it alters with road speed it might be something as simple as a tyre catching on something or something as dodgy as a knackered gearbox.
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Unless the owner could show me what the whine is I'd probably walk away.
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