tranny prob
 

i have a dseries motor.when i shift to third gear at high rpm's i get a grinding noise,but normal driving i don't get it.anyone know whats going on?

synchro are going.......time for a swap!!!!!

Try switching your tranny fluid to GM Syncromesh or don't shift so fast.

where can I get some of that GM stuff?

GM dealer, but you might have to order it because most dealers have never heard of it. Also, make sure you get the correct kind that says "friction modified" on the label. I've posted the part # before so just do a quick search. It's about $14 a bottle.