When I hold the blazer at a stand-still in any gear (R, OD, D, 2, 1) with the break applied I can hear a rattling vibration noise from under the car. It sound like it comes from under the passenger seat area but under the car. It's not there when I put it in park. And goes away when I start driving. Any suggestions on what this is? Something loose maybe?

 

Got it figured out! My catalytic converter was loose! The joint against the muffler was very loose and needed tightening!

 

good to hear it was something simple.

 

Mine is doing the exact same thing. Its not really a metal to metal noise though. was yours?

 

Mine was making a funny non-metallic rattle and my cat was cracked. If you can't find anything on the outside making the sound, bang on the heat shield with something and listen for a rattle. If that happens, then you need a new cat.
You can go ahead and ignore it for about two years, but the chunks will eventually plug it and then you'll really be in trouble.
When it really gets bad, all sorts of weird stuff will happen, first you'll notice that the shift points go WAY up and there is no power to ever get to them, then when it's totally plugged, there will be a hiss and a banging rattle at WOT and you'll be able to go about 40 up a hill if you floor it, but see no symptoms below 3K. Won't throw a CEL either!
It's real fun to have that when the only merge ramp in town is a short one!

Hope this helps!

Regards,

Thomas

 

sounds like its in the dash though.