I have an 01 expy with 83K on it. when I'm getting on the highway and get up to cruising speed i hear a noise that sounds like im running over rumble strips from the rear of my truck when it drops into low gear?
I am totally illiterate when it comes to working on cars. Can anyone help me?

I love this truck but it's turning into a money pit and I don't want to get ripped off on repairs.