2000 blazer Lt 4x4 automatic trans. Early today I was backing out of a parking spot and when I shifted into reverse hit the gas and nothing happened. It didn't engage into reverse so I put it back into park then back to reverse again. Hit the gas a bit and nothing happens again for about a second and then it slams into gear and lurched backwards. Now when I drive it around it makes this noise I can't really describe other than a whining noise. It only does this in first gear. After it shifts out of first the noise instantly stops. Also the transmission seems to still be working just fine it shifts fine and everything seems normal other than the noise it's now making.

 

Sounds like a sunshell problem. A known weak link in 4L60E. Mine went out too, was not abusing it even a little bit. Plan on a complete rebuild and don't drive it if you want it to be rebuildable. Also flush trans cooler and lines to get all the metal out in any case.

 

Rear planetary gears that are damaged will give those symptoms. May want to drop the pan and take a look for metal.