I have a 2002 blazer that is making a thumping noise when I drive in 4x4. I have replaced the vacuum switch and Im replacing the transfer case motor today. It has an New process 233 transfer case, Im not sure what the front end and rear end are. It does not always make the noise it just happens kind of randomly. Its almost like I hit something but when I look behind me there is nothing. It has push button 4x4 and it only has the 3 selections 2hi, 4hi, and 4lo. It has never made the noise in 2wd. also once or twice when I have put my truck in park after having it in 4wd(I put it back in 2wd before I parked it) the truck rolled backwarks until i put my foot on the brake, then when I let off the brake it was fine. My drive way is on a slope and it rolled most of the way down before i stopped it. Ive had it at 3 diff. shops but they dont know what it is and they sent me to a dealership. I went there and they said its the ball joints that are making the thumping sound, but with the frontend on jack stands it doesnt have play front to back or up and down. Ive done all I know, I will be thankful for any suggestions.

 

Did any one have any suggestion or a link to thread on this
We are having a problem like this but was wondering when we put in 4x4 you will hear a thump ,it never really goes in 4x4 ,put new ball joints on last year and have rechecked them , could it need a new servo