Crazy noise....need help!!!!
 

i have an 81 gsl with a 4 port 13b and a 45 DCOE weber. i keep hearing this crazy noise that sounds like it is coming somewhere around the carby area of the engine bay. it only does it when i'm really getting on it at more that half throttle in fourth and all of fifth when i'm accelerating hard as well. the only way i can describe it is like a piston engine sounds when a rod is knocking really bad at high rpm's. The other thing that is stumping me is if it's making that noise if i breathe off the throttle pedal to less than like half throttle it will go away until i get back on it. let me know what you guys think, i have been stumped on this one myself.

Thanks in advance for any help