Nailing is a much louder than usual "pinging" you usually hear. The springs inside the injectors may weaken over time allowing fuel to be sprayed earlier than required. The new CDI diesels are able to pulse electrical injectors very quickly and for multiple times over the combustion event to almost eliminate the diesel ping.

I drove a new CDI a couple of weeks ago and it was really quiet. Although you you could still tell is was a diesel.

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)