I have a 99 z71 with the 5.3 and I have never really noticed this until i got exhaust in July. I thought i could see the tach moving up and down a little bit but the exhaust made to where i could here it, so i know the exhaust didn't do it. It was one of those things that i would turn the radio up so i didn't have to hear it but now it's just aggravating. When driving only between about 45 to 55 my truck almost make a sound like im peddling. It doesn't feel like transmission is slipping or anything, it's not real fast just a lope. I did a motor swap with a 04 5.3 after blowing up the 99 motor and it still does it. I don't know what it could be and sorry for this thread being so long just trying to give detail. Thanks for the feed back

 

did you swap the PCM with the motor sway? other common causes can be a dirty throttle body and bad O2 sensors. clean/replace is a cheap attempt to fix. Some motors like certain O2 sensors versus a universal one. hope this helps.

 

No its the same PCM, the throttle body is definately clean because i had to use the same throttle body and intake because the 04 had an electric throttle body, so i cleaned the hell out of it. Is there anyway i can test if the PCM(this is the computer?) is bad or the 02 sensors are bad before i go replacing them.