Thanks mudnut! I've just read that article and found something you mentioned to ring true with my experience at idle after cruising down the highway:

"The constant knock of the diesel at idle contributes to the problem."

After purchasing my RD28 I had taken it to a mechanic for a general service and mentioned my observation but he couldn't diagnose what it was. Is the knocking an indication of something abnormal that I should look out for? I do have a fuel pump leak too. Cheers