well my 86 sdl has a loud knock at idle also.when cold you don't notice it as they are all knocking,but once it starts to get warm it settles down to just one.i've cracked inj lines and have isolated it to number 3,i've changed delivery valve seal,swapped inj's with other cyl's.been told by someone on here that number 3 is the worst for scoring pistons and that that is probably what i'm hearing.i do find this hard to swallow as once off idle say 8-900 rpm's it's quiet.i also have an 82 gmc suburban with a 6.2 and it has a LOUD knock on one cyl but it's been doing it since the engine was put in,it now has almost 100,000 miles.so the knock evidently doesn't bother the engine,just the driver.at least it's only one cyl.unlike a mid 90's powerstroke ford which knocks on all 8.