I have this exact issue on my 2004 Z71 Tahoe 5.3. Although, in addition, my RPMs seem to increase +/- 300 when I'm cruising around 50mph with steady pressure on the pedal. Have been to two dealerships in NW Arkansas and neither have been able to diagnose anything because there is no code. Granted, I've got 95k miles, but I have a GMPP Majorguard warranty until 105k miles. BOTH dealerships have turned me away because, with no code, the rough idle is "normal". Additionally, my truck nearly stalls out at idle, especially when I come to a stop or when I'm turning at low speeds in a parking lot. In fact, last time I picked it up at the dealership it died out when the tech was pulling it out of the service bay.
This is something that only recently developed in the last 2k miles. I have cleaned the throttle body, MAF, and filter. I have had a GM fuel injector treatment at the dealership. I've tried BG44k per the dealer's request. I've replaced spark plugs and wires (delco OEM). I've had the fuel pump and tank module replaced. I've had the intake manifold checked for leaks (went beyond carb cleaner spray and had it hooked up to a 'smoke machine' to be tested). I've replaced the battery and alternator.
I'm at my wits end and having taken it in and paid almost $300 for diagnostic work (only to find nothing) when I paid for an extended warranty is just ridiculous. Neither dealership will even look at the truck again because they say "No code? No problems! Must be just 'normal' for this truck."
But it's not. I drove it for 30k miles when I bought it from the original owner without a single problem.
