Come to find out after researching it a bit, the 2010 & 2011 models were under a transmission dipstick recall. This is almost certainly my rattle! I'll search again tonight but the mounting tab must be missing on mine.
Recall info:
Subject: 12138 ? Transmission Dipstick Tube Fracture
Models:
2010-2011 Cadillac Escalade, Escalade ESV, Escalade EXT
2010-2011 Chevrolet Avalanche, Silverado, Suburban, Tahoe
2010-2011 GMC Sierra, Yukon, Yukon XL
Equipped with Active Fuel Management
*******THIS PROGRAM IS IN EFFECT UNTIL NOVEMBER 30, 2014.*******
Condition:
Certain 2010 and 2011 model year Cadillac Escalade, Escalade ESV, and Escalade EXT; Chevrolet Avalanche, Silverado, Suburban, and Tahoe; and GMC Sierra, Yukon, and Yukon XL vehicles, equipped with Active Fuel Management, may have a condition in which normal engine vibration while in AFM mode may cause the transmission dipstick tube to fatigue and fracture. If this occurs, it could cause a tick or rattle noise from the front of the vehicle.
Correction:
Dealers are to inspect the transmission dipstick tube for a fracture and replace it if necessary. If no fracture is found, dealers are to install a foam sleeve around the tube to absorb the vibration.
Parts Information:
Parts required to complete this program are to be obtained from General Motors Customer Care and Aftersales (GMCC&A).
Parts Ordering Information ? For US & Canada Dealers Important: Involved dealers will receive RIM "Service Campaign" Stocking Policy Proposals for the transmission filler tube, P/N 24260606, based on their past sales on this part. The RIM proposals will be scheduled to begin October 25, 2012. Once the policy is approved, RIM will push this additional stock accordingly. Due to the predicted low failure rate we strongly encourage that you allow RIM to manage your inventory on P/N 24260606. Canadian dealers will not receive a special stocking policy but are also encouraged to have RIM manage their inventory.