K-5-Titan

ok i have an 83 k5 blazer, 350 stock engine 400 turbo tranny brand new. well one day i was pullin away from my job and it was pretty cold outside but i let my blazer warm up as usual for like 5 minutes. it made a super loud knocking noise and has been doing it ever since..but it only does it on take off when.. i thot it might have been a piston rod loose but if it was wouldnt it be doing it all the time? not sure wats goin on cuz when im going down the road and floor it it doesnt make the knocking sound at all only when i take off from a dead stop. its been doing it for a while now and iv checked to see if anything could be rattleing or hitting something so idk. sounds like metal on metal knocking. and once it gets warmed up really good like iv been driving it for a while it doesnt do nothing even on a stopped take off its got me stumped. any ideas???? thanks.