Knocking noise

I have an 1998 Chevy Astro van. Lately when I hit a speed of 60 it makes a vibrating noise. But only sometimes. I’ve taken it to a mechanic and they said they can’t find anything wrong with it. Any thoughts. It’s like a vibrating noises that can be felt in the whole van. I’m not rich and have taken very good care of my van and do not want to sell it. It also only has 70km on the engine. 4.3 engine v6