Noise ... raddle

I wish I own my Hummer when you asked this question, on my 04 as well 03 H2 ran in the same issue, its a simple but will drive you nuts to fix, maybe this will help anyone with the same issue now. First you have to remove your shifter handle, use a 2nd to the smallest Allen wrench to remove the small nut on the shifter, make sure don't completely take it out, because its small and you can loose it easy while unscrew that nut, once your handle is loose, removed the handle, than in your center console, there is 3 to 4 bolts depends how many is there has to come out, than once remove the center console box you will see 4 more bolts you have to remove, one of the bolts is on the middle, it connect the base to the body closer to your shifter, than you also need to remove the frame that is holding your AC unit as well your radio, I did not take that off but loose enough to pull outva little so you can pull out the center console that is clip under that frame that's holding on your radio and AC unit, once the frame is loose enough you can pull it out slowly so you wont break the plastic frame cover, its nightmare, once you removed the center console frame, your main shifter unit will be that whole shifting box connected to your shifting pole. On the right side by the main unit, you will see 2 thing, one issue is the connecter with 5 wires that goes in the shifting module is been either cut or loose, because Hummer use a such thin wire and as the time goes by each time you shift it either pull the wires up or down because it does move that connector with it, and those wires some will be pulled out with shifting movent out, and somewiir the movent will take its toll and some will be cut in half due to Hummer use plastic to make everything. You could either run a thicker wires and make sure its long enough so its not too tight, that is how those thin wire break, or you can go to Autozone and get a new connector I think is call the Pigtail standard 6 prom, or called shifting solenoid pigtail connector, its already set up, and 5 time thicker wires, just cut the old connector one wire at a time, so cut one than attach to the new connector so you dont get the wire mess up. Than plug it back in and fixed. Also trick to manually unlock your shifter so you can use your H2 before you can fix it, same way to get into the wiring connector by your solenoid, you will see kind of like a button little to the right just under the connector, push that in and shift than release the button, only time you need to shift from park you have to push that button to drive your H2. When putting back your handle, make sure you shift to Drive or lower gear, than put on the handle until it stop, tight the nut and your done, because the handle has a small spring, if you push in the handle than you can push that button on the handle as well if you put too loose, so best way is put it in D or 2 gear, and drop that handle on the pole and enough that you starting to but pressure but stop there and tight it up you should be good, if this long message will help anyone with the same issue.

The technique I always used to identify where such a noise was coming from was to use a very long screwdriver (2 feet long), while the engine is running. CAREFULLY touch the screwdriver end to each pulley or other object under the hood, and put your ear to the handle. The device making the noise will become very clear.

It could be AC pump/clutch, alternator, water pump, power steering pump, etc.

Brake switch is bad. Check to see if you have brake lights. Brake switch is under the dash and it touches the brake arm.

Chances are that a heat shield located under your exhaust or the catalytic converter has broke loose and rattles when you are idling in drive. This is common on older cars and is only usually noticed while idling in drive because the engine is working under a slight load and runs a little rougher. A good exhaust shop could probably fix it for very little.

Do the brake lights work?

sounds like the cable shift has come apart but without my head under the bonnet i honestly couldnt say ,never knew they made a auto version of this car as never seen one here

If it feels like the whole car (dash & steering wheel area) is shaking at idle in drive then it is often a bad engine mount causing this vibration throughout the car. An actual isolated rattling noise from under the car could be the catyalitic converter if it sounds like its in the exhaust system.

the shifter release solenoid is at fault located inside the console

if the noise only happens when the vehicle is in motion...check the front suspension. place the jack under lower control arm ,then get a bar and lift up on the tire..ball joint issues are well known in blazers and cause this condition. if it does the rattle noise when its idling and to op temp--could be the cat converter causing the noise on pass side directly under passenger seat. when cold, smack the cat with rubber mallet and if any rattle is present, cat should be replaced so it dont cause further damage .hope that was helpful !!

If it is a Canadain built vehicle the law required the head lights to stay on while the ignition is turned on. I have owned 9 Jimmy and Blazers including your year of Jimmy and none of them ever amde a noise in the dash when I put the vehicle into park. The shifter only has an electrical line runnig to the light control and a cable which runs under the carpet to the transmission.