ive been chasing this sound have changed front and rear wheel bearings and cv shafts but still it persists. usualy happens at 30mph or above and i cant make it happen. defiantly sounds like a rotating part. the u joints dont seem bad and both diffs are full of fluid

 

Just a thought but maybe your fan clutch.

 

Most likely the fan clutch as TMM suggested.

 

Is there any way I could test it I'm not familiar with how all of that works I know it doesn't with the heat on with the heat off at very random times

 

98gmcjames: your reply is leading me to think that you have fan clutch confused with blower/HVAC fan. There is a way to test the fan clutch....suggest doing a search on YouTube for videos; just fan clutch not Blazer specific.

Now if the noise only occurs when the blower fan is on, then forget the fan clutch, problem is with blower fan.

 

My fan clutch would be noisy especially on a very warm day and the engine was at full operating temperature. Then when I would take off from a stop light, it would roar loudly like a semi truck in an extra low gear. I got a new Murray fan clutch for about $53.00. The hard part of replacing it is getting the right wrench for that large nut ---36MM?-- on the fan. I rented a set of 5 wrenches for 71.00 and returned for refund , of course.

 

so i think we found it, i believe the front diff isnt completely Disengaging So I pulled the vacuum switch off of the top of the transfer case in the ball was stuck halfway in the line drained out Black fluid But unfortunately the noise returned the following day So now I'm not sure where to go