I have a 2009 Hummer H3. When the engine is started cold I get a loud rattle and the car shakes until it warms up. It appears to be once the oil pressure goes up the rattle stops. I have had it into 3 different dealerships, 1 a former Hummer dealer with hummer techs still on staff and no one can figure it out. I have tried for almost 2 years to get this fixed and because it only lasts a few seconds and does not throw a code I am told they cannot figure it out. I have to pay several hundred dollars out of pocket each time to have it come back the same way. I have contacted GM and they mentioned having a field engineer go to the dealership as my vehicle is back in again for a week and a half and counting but the dealership said getting a GM Field Engineer out is rare. So why do they call them Field engineers if they do not leave the office I asked. So I have an extended warranty that will not give permission to tear the engine down to fix it and if I have to pay for that it is over $1000 to break it down I am told. I have mentioned it could be the timing chain tensioner but they will not tear into the engine to see. I have owned 3 Hummers and my son has one and the other ones never had an issue like this. So they are on step 1 doing an oil change to see if the GM oil filter will solve the issue. I doubt it since I only get my oil changed at the dealer or one local place. I miss the days of no codes needed when mechanics were real and could find an issue on their own. Now, no code, no way they can fix it. GM used to be a good brand but the customer support is terrible unless you own one of their new cookie cutter cars.