a knocking sound underneath my car in cold weather. Tachometer is steady .

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Knocking sound coming under the car, don't know where coming from, if the weather is warm or hot i hardly hear the sound, but in cold weather, the sound becomes louder.The knocking sounds only if the car is in motion, but on neutral idle no knocking sound . I have a mechanic check and he diagnose it need a new motor mount. is that true.