I have a 2005 yukon xl . I owned it for 5 months but it makes a thuding noise under neath ?

Sounds to me like a transmission mount or two is torn...get a floorjack with a woodblock under that one and try to jack up the the whole assembly...that way the torn rubber would show itself~

where is the transmission on my rig? How big is it? How many are there under there.. The sound moves ..

between the engine and the driveshaft is the tranny, it has some shift linkage on the side and a vacuum modulator hose on the bottom~

could be the motor mounts as well, you can find those between the block and chassis, you could put a jack under there and lift by the pan with a wood block and look for splitting~

the pan is about 15" square, the mount is about 3" rectangle with captive connections so when the rubber does go to hell you end up a thudding noise underneath as you describe~

If there is no visible damage try checking the u-joints on your drive shafts and the front cv's going to the front wheels. Does the noise change when you have it in 4wd, try turning while moving. Most factory u-joints are now non-greasable and require replacement once worn out. Look for play or siezed u-joints.

Trans and engine mounts are most noticeable during hard acceleration and while engine braking(down shifting your trans to slow down). The trans mount will jump and slam on the flour while accelerating. It is generally just one thud. With a bad mount.... start with the u joints as big foot said.

General motors transmissions haven't had any vacuum lines running to them since fuel injection in 87... for future reference

They used the turbo 400 in some 3/4 ton/1 ton and rare v-8 1.2 ton W/T series trucks vehicles beyond 87, it has a vac hose But nothin that new.

My bad the 700R4 had vac lines up until 93 on select few models then it all went to 700R4E aka 460LE

I believe the 700/4l60 didn't have a vac line. So you were close in the first statement except for the use of the 400 untill the 4L80-e was attapted for the HD application later in the 90's

well whatever it's got, may have disconnected...that's the point. sheesh~

No when it does it... It is when you are going 45 to 50 miles per hr. and it will .. thud thud thud.. thud.. thud... worse when you are next to like a divider in the hwy.. or sounds worse. so I guess I need to have my mounts and u joints.. it is 2 wheel dr. not 4... thanks all...

the noise is toward the back, sound like anyway...

...like others have asserted, it's the U-joints....there are needle bearings in them what go to hell giving the condition that you describe~

thanks everyone... getting it in to be fixed and will let you know for sure what it was....

Thats more likely to be a ujoint has broken. $10 part at auto store. I've replaced it more than once.

watchout for those "french locks" which need a needlenose pliers to bend out of the way~

not u joints.. checked.. they have looked at everything. still noise at about 45 to 50 mi. per hour. some hear it in the front , some back. still there after checked several times.. bugs me..

have you figured out a fix for this problem? mine is doing the same!

No I never did.. I am driving it.. Had things checked out.. they found nothing... so I just drive it and ignore it.. sometimes it won't do it.. and then especially like down the turnpike next to a wall or something. it is bad...

Joseph do you have the same year and all??

oh I see, hm. Yeah I have a 2005 GMC Yukon XL SLT 4 wheel drive. I'm gonna keep working on it when I can, at least it drives for right now.

It's probably the parking brake if it's coming from the rear end. My rattles every time I hit a bump. My buddy who's a mechanic said just rip it off lol unless you are going to be towing something you don't need it

Did you ever figure out the noise

Kind of sounds like when my front hub was going out. Sometimes it sounded like the drivers side and sometimes we thought it was coming from underneath but it ended up being the passenger side hub assembly. It has pressed bearings in it. When I finally got it off it was galded and it just kinda fell appart