likely you have a worn accessory belt tensioner. they are available new. could also be worn engine mounts (very likely in a 10-15 YO diesel car). loos accessory belt will cause all kinds of funny harmonics. when you turn the a/c on, the ECU increases the idle a couple of hundred rpm, and also the extra load on the belt will cause the tension side of it to tension up a lot, which can stop some vibration.
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