Truck Repair: knocking under load?, sierra 4x4, gmc sierra

Question
SO heres the thing, I know that this might no be in your area of expertise but any insite would help... I have a '93 gmc sierra 4x4 with a little v6, it is a standard and it seems that when I am accelerating weather it be up a hill or taking off from a corner the engine when under that load knocks and sounds like a diesel engine.  As soon as i take my foot out of it the knocking stops.  This is also true after u have finished climbing the hill.  What could be causing this?  And is this something I can fix?  

Answer
first thought on this would be cheap gas... if you think your mech. incline you could try to retard the timing about 2 to 3 deg. this would let you run cheaper gas. with this truck you should turn the dist. counter clockwise.if this condition isn't fixed soon you could burn a hole in a piston. thanks for the ?