Ive had my 99 trail blazer now for almost 2 years and outta no where I started getting this knocking now under the driver side floor board and over the past couple days its gotten worse. Now it progressed to grinding and more frequent if I go over 25mph. Took it for a free inspection (drive and guess) and they said I have to replace my transfer case, 2250.00, Not possible on unemployment and single mom, any suggestions on possible other less expensive reasons????? Any help right now is greatly appreciated, Ive been car less for 5 days.. Thanks

 

*Moving from 1st gen Tech to 2nd Gen Tech* - The 1st gens ended in 94...

From the sounds of it, your transfer case likely had a leak which caused the fluid level to drop. The NV236 transfer case that all of the Trailblazer trimmed Blazers came with uses a special fluid and is very sensitive to fluid quality and level. Check your fluid level and probably flush it out. You will need 2 quarts of GM Autotrac 2 fluid to fill it properly. 2 more quarts if you want to flush it again afterwords. If it is making that much noise, it likely is toast.

I have also seen the front driveshaft on these trucks wear out so that is another thing to check.