You should take a look at the "how-to" on stopping the "BCU rattle". There are several control boxes mounted under the dash which can knock together or against the bulkhead and amplify any chassis vibration. These sounds can be transmitted by the air ducts and appear to be coming from other locations. Application of padding will cure this.
Diesel vibrations tend to be greatest at around 1800 rpm. Was your post a typo?
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