I have a 1979 K20 suburban with a 350 and manual tranny. I've been doing a lot of work to it lately. I just reset the distributor timing and adjusted the fuel/air mixture on the carb (it's the stock Rochester Quadrajet) and by ALL means it's running much better than it ever was. It was running WAY too rich and it was smoking, it would sputter and backfire sometimes or stall out, and it was generally difficult to start, especially when it was cold outside.
Virtually all of those problems have been eliminated, my fuel economy has drastically increased (I'm not sure by how much but I'm guessing by about 4-5 mpg, putting my current fuel economy at about 14 mpg vs. 8-9 I was getting). Throttle response is much better than it was (but it still has that irritating lag that all quadrajet carbs have at initial throttling). The truck never has any problems starting anymore and fires up right away instead of needing to crank for 30-40 seconds while i pump the throttle every time.
but the only problems I've noticed since the tune up are when I'm at full throttle, I hear a rattling noise coming from somewhere near the transmission. I'm almost positive it's not pinging as it doesn't sound like the engine is knocking, and I hear the sound inside the cab of the vehicle and have not been able to hear it when the truck is parked and I rev the engine. Only when driving at freeway speeds.
Also, since retuning the timing and carburetor the engine has been idling at about 1300 rpm, as opposed to the 900 it should be sitting around, and it sticks there while the truck is in neutral until I tap the throttle and then it kicks down to a normal idle.
My first thought is that perhaps there is some varnished fuel clogging up the carb and the meter plate is not transitioning from medium throttle to idle properly. Any thoughts on this???

check the throttle cable and all the linkage. you will find something in a bind or durty cousing it not to go all the way closed.

check your heat shield on the cat for the rattle. most likly its the change in the wind under your truck that is making it move.