I'm going through this myself and found this thread from 2010....
I've got rattles coming from the passenger seat and rear passenger side seat. I've removed the headrests for diagnosis and it wasn't those.. The only thing that makes the rattling go away is when someone is sitting in those seats.
When going down a gravel road, I noticed that the plastic interior door lock mechanisms rattled but I found a fix for that on YouTube which I've posted below:
https://www.youtube.com/watch?v=-wll8HaRytM
The little HVAC sensor fan in the headliner above the driver seat is starting to drive me insane because of noise when it's cold at start-up. My two previous trucks had this same issue and I've never dealt with it. (It's always gone within 2 minutes of start-up so not on my mind when I get back to the house.) I did find this statement from Chevy for NBS trucks and they used the same part on the newer trucks:
"Document ID# 1639237
#PIT3511
Condition/Concern:
Dealer or customer may comment that they have a buzz or vibration noise emanating from the headliner in an area that looks like microphone or a speaker.
Recommendation/Instructions:
The plastic escutcheon grill, which is round and about the size of a half dollar and located just above the driver to the left, is the interior ambient temperature sensor. The sensor is used in conjunction with the automatic HVAC (C68). On occasions, this sensor has been known to make a buzz or rattle sound. The sensor uses a small fan to pull air over the sensor. This buzz noise may be intermittent.
Although it may not be possible to alleviate all of the buzz or noise from this sensor, Engineering has found that if the sensor is installed incorrectly that it may bind and cause the fan to make a buzz noise. Remove the sensor and while doing so check for any binding. Reinstall the sensor and re-test. If the noise is still present, the sensor itself may be noisy and require replacement."
The seat rattling is getting worse so if anybody has resolved this issue, please....for the love of God.....please let me know. I did find this, but no final remedy:
http://www.tahoeyukonforum.com/forum/showthread.php?t=8770&highlight=rattling+seat
Thanks!