wow that^ made me look stupid

  yea  a little

if its doing it only at 3500-4000 when your not really pulling anymore but before you let off to shift then its more than likely wristpin slap.  not a good thing but not as bad as a rod knocking.

what oil are you running?  you need to change the oil VERY often if its black change it.  i had a celica years ago when i got it the oil was black and gooey.  i changed the oil ran it for ten minutes with new oil and it was black again. i ran it a couple days then used some "Rislone" to cut the sludge then changed the oil again.   it lasted about a week before it was nasty black again.
if a motor is run till sludge builds up bad then that sludge has to be cleaned out.

when you change the oil you need to go up to 10w40 unless thats whats in there now but i bet its not