I have a 99 GMC Sierra and today I went offroadin' with some of my friends, I turned on the 4X4 when we were in the river in the sand to get a little more traction. When I turned it on there was this thumping like noise, it would be like "thump thump thump", I think it was 3 thumps then a small pause and it came back, it would thump more often when I would go faster. When I turned the 4X4 off the noise was still there, when we got back on the road the nosie kept going but then went away after about 2 miles. I thought it completely went away but before I cam home I turned the 4X4 on again and that noise came back. Does anyone know what this could be?

Thanks.

 

you have autrac i presume? well if you do i beleive you have an actuator that locks the front differential in, that could be what it is, ive seen a few of those go out.and sometimes the front diff actuator sticks and it takes a minute or two, or sometimes a lil driving. and ifyour sitting still and its still doin it that it trying to engage, id play with it a lil bit , if still no luck id say bring it to a 4x4 shop or a gm dealer. i have a manual transfer case and have never had a problem, i love it. also another things but doubtful, one of your front hubs or your cv shafts might be stripped out/broken..just my 2 cents

 

Alright thanks, I'm going to a 4X4 shop today and see what they say.

 

Thats weird, before I went to the 4X4 shop the noise was all there, but when I got there the noise was all gone, even when I turned the 4X4 on because that is when it makes the noise, nothing. All gone! But on the way home my truck just suddenly turned off, twice, but started right up. First it just went off, turned right on then like 10 feet later turned back off, then I stared it back up? Wierd stuff.