I have a 1985 GMC Sierra that has a spring sounding like ping

O.K didnt know if your answer was meant to be funny or notcause me saying spring sound. But the hubby says no.I replaced plugs yesterday. An messed with idle a lil. Truck seemed to idle better an smoother. But messed up more at 35 mph an wanted to die out when I tryed to back up an pull out at a slow speed, but also not so much of the ping at 55mph, adjust idle back down an ping got steady again at 35 mph an countinued up into 55mph didnt want to stop pinging, ping almost sounds like it is low drivers side close to fire wall at feet.

Sorry I think I was using ping to shorten spring sound.I should of known better. More or less catching spring. Idk about the code thing, all plugs an wires are new I did plugs yestrday and wires a months ago due to engine fire, so distrubtor was also worked on then. The guy my hubby took it to to finish the
wiring thinks maybe a flat cam but not for certain, I am just not sure if I feel comfertable with him pulling everything an charging a arm an leg for somthing he is not certain about.
It is a 350 c.I engine, truck was converted from diesel to reg gas. We notied the issue about a month after buying it, to my understanding truck was completely rebuilt a lil over a yr ago, and scence buying it only thing I have done is replace the hermonic balancer and swaped out the carb after we started having this problem, at that time I did notice prob was not as bad but still there, but then we had a back fire through that carb and engine fire, so now got the old carb back on. But all plug wires, hoses, fuel lines, vac lines are new, and most the elec wiring, only other thing that I know the hubby does diff with the truck is the fuel, the other owner used higher octain fuel an hubby uses the 87 I do belaeve.I think that pretty much covers all I know about the truck. And TY for your help and time with this.

They did the distributer an timing when finishing the wiring so ill be a lil annoyed if its off already but got to admit been wondering myself, could explain the slight missing I noticed the past few days and the poping sound through exhaust.I will start with gas an timing again and go from there I guess.I have been saying we should try to go with higher octane fuel but hubby is pig headed, Like I said his father ran the high octane befor we bought the truck.I am not sure on the compresion ratio.I love working on older cars but this truck will be the death of me. Lol