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Thank you for that information. There were known issues with the H3 heads with vin #'s that were below 287591, your H3 is within the range of effected vehicles. GM issued a notice to dealers about the valve seat issue and they were repairing them under warranty.
The good news is I have seen GM repair these even when out of warranty as they were worried about a class action law suit. Considering your low mileage there is a very good chance they will cover it. They may make you pay a diagnostic fee at the GM dealer to make sure that is the problem and then they will either do it for a reduced price or free as a "good will" repair.
I would first call GM hummer warranty assistance. Let them know you are aware of the issue with the valve seats. There contact info is below. If I can assist you further let me know.
GM HUMMER Warranty Customer Assistance Centers are open Monday through Friday, from 8:00 AM to 11:00 PM (Eastern Standard Time). To contact the GM HUMMER Customer Assistance Center, call: 1.800.462.8782