Hi, bit of a strange one this. When going over rough ground my 99 4dr RHD Blazer has developed a loud knocking sound from the front left side - but only when backing up? I have gotten underneath & grabbed every component I can get my hands on & tried to shake it around but I just cannot find any significant movement in anything? It is quite a loud knocking sound - any suggestions/advice would be appreciated.


Oh - and before anyone says anything, I have checked out the hood hinges & they are fine, I fitted the GM washers to them a couple years ago which cured the knocking/rattling problem I had at the time.

 

Is this in 4wd drive when it knocks. Mine did that and it was the front driveshaft hitting the cross member. Shimmed the tranny up and fixed it

 

Hi, doesn't make any difference whether in 4wd or 2wd. I have tried it all. Thanks for the suggestion though.

 

CV joints?? Engine or trans mounts??

 

I think I may have found the problem. I went under the vehicle again earlier & found a little bit of looseness in the joint on the front driveshaft at the transfer case end. Does anyone know whether this joint can be replaced on it's own or do I have to buy the whole shaft? Even if it's not the source of the noise I am not happy with play here so feel it needs to be replaced.

 

The U joints on the front drive shaft are replaceable...if that is what you are referring to.

Be advised if the U joints are original, then they are "glued" in. But not a problem...just apply a torch to the end and watch the glue ooze out of the small hole in the ends. A bit fun to watch...just do not let it drip on your foot.

Then use a U joint press to remove the old and install the one U joints.

 

The looseness seems to be in the CV-type joint on the transfer case end of the shaft. I am not 100% sure but I think RockAuto supply just that joint on it's own?

 

Just a little update on my front end rattle that has been bugging me for a long time now - thought i'd share it in case other owners have a mysterious knock from their front end.


I found quite a lot of looseness on the CV joint on the front propshaft, the one nearest to the transfer case. I managed to obtain this joint through RockAuto & it was a pretty easy job to replace. It has made a hell of a difference, a rumbling sound + some vibration I have had for quite some time has finally gone. But - I still had a slight knocking there!


So - put the front end on axle stands & got underneath & went through everything yet again to try to find any looseness. I checked balljoints, CV axles, hub bearings, steering linkage - everything I could think of but found nothing.


Almost at the point of giving up I decided that while the truck was up in the air i'd do an oil & filter change as it was due for one. This is when I had my eureka moment. When loosening the oil pan drain plug I caught the undertray under the engine with my wrench - and it was loose! A couple of the torx bolts holding it on at the back needed tightening. Couldn't wait to go for a drive to see if the noise had gone - and it had!


Just thought i'd share this as a bit of useful information for anyone that has an elusive front-end rattle.