I am the owner of a 2012 suburban 5.3l 4WD and I don’t know where else to go but here, I have a thud or clunk that occurs every time I accelerate from a stop, it only happens when I accelerate normally or hard, and doesn’t happen when I ease into it super slowly. I’ve been told it was the slip yoke and that it needed to be lubed, so I tried that and the problem wasn’t fixed. You can also feel the thud right at your feet on the drivers side. Another thing to take into account is that there isn’t any clunk when shifting from reverse or drive so I don’t think it is the u joints Anyone have any clue what this could be?

 

you may have a worn u-joint, which has developed slop. That slop gets taken up when the driveshaft changes direction such as going from drive to reverse, or vice versa, with a clunk.

 

Check you drivers side motor mount. Hood open, foot on brake, give it some gas in gear and see if motor lifts up.

 

Did you ever figure out what's wrong? My '09 Suburban 5.3L 4WD has the same exact issue. It's actually had it for a while. I just rebuilt the entire front end (tie rods, ball joints, control arms, struts, sway bar end links, sway bar bushings, wheel bearings) and the clunk is still there. I've had new motor mounts sitting in my garage for months... Never bothered putting them in because if I do the test my motor doesn't lift up like all those YouTube videos, so figured it wasn't worth the hassle (4WD models require dropping the front differential to install mounts). If it is a bad motor mount my motor isn't moving much.

I've read the TSB's about the slip yoke sticking and producing a clunk, but there does not appear any long lasting fix (assuming that's the problem).

 

Update: Bit the bullet and changed the driver side mount, which took about four hours. It was broken clean off. The clunk noise is gone.

What does not make sense to me are these videos showing the engine lifting up 4-5 inches when they do the brake test. The mount has a safety loop so the engine can't raise more than about an inch. This is why I never assumed it was broken. Are people just driving so aggressive they break off the loop safety loop on the mount?

 

Great that you found and fixed the problem.

As to why your engine couldn't be lifted high enough - I have no idea (we have a 2014 Yukon Denali AWD).

 

Yeah, I'm not sure.

This is a screenshot form a video I took of my broken mount after removal. This is about the max it can physically move because there's aluminum casted into mount with a cross pin (about one inch total movement):



I can only assume all those videos (like this: ) are people who have physically broken the built-in safety of the mount.