Nate:

If the knock goes away off throttle coasting and comes back with fuel, it is most likely injector related. Check for diminished or vanishing knock with the cap nut loose -- this is diagnostic for injector knock.

you could also have bad IP pressure valve seals, but this usually causes rough running at low speed, too.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!