I have a 1996 B 2500 with 165,000 miles and a 318 with an automatic trans. Occasionally it will make a noise like you're running over the rumble strips at the edge of the highway. When it first happened I thought that is what it was; only I was in my lane. It stops as soon as I let up on the gas. It may not do it for another 250 miles or it may happen again in 5 miles. It has been doing this for about 2500 miles. Been driving Dodge vans for 35 years and this is a new problem for me.

 

Is it just at highway speeds when this happens or at lower speeds too? Do you feel the vibration in the wheel or more in the seat or passenger seat? I'd think it good to check u-joints for slop but seems if bad they would be making some noise at any speed and all the time....
Have you checked the motor mounts and transmission mount for excessive movement? Had read a couple articles on symptoms of bad motor mounts and the excess vibration and noise is 2nd to metallic clunking and banging sort of sounds in gear or accelerating.... it's easy to check and something to begin with.

One of my bosses has a 2000 3500 5.9 and has described the same sort of thing as you, saying occasionally it feels like he's driving over a patch of lane markers... couldn't figure what the cause is but I'll ask tomorrow to see if he figured out what it was on his yet. His pick-up sensor in the distributor went bad and and when we replaced it. We had thought maybe the vibration was the sensor failing and cut off fuel intermittently causing some engine misfire, but apparently the vibration and noise would still come and go

 

It mostly happens at highway speeds, but it did happen at around 40 once. Haven't checked the motor mounts yet. I don't think it's the U joints as there is no clunking and I think that it wouldn't stop when I let up on the gas and go for miles before it happens again. I'll be looking for what your boss found on his. Thank you!

 

Bad u joints can do all sorts of funky things. Don’t rule them out without checking them.

 

I'll do that. Thank you

 

Well.. I asked my boss about his vibration and told him about yours. He says your description sounds exactly like his. He's a pretty good old school type mechanic but has no idea what the cause is yet on his. It's been running well except for still getting that random vibe and noise thing. Said he was going to finish off the tune up stuff (plugs-wires etc.) then go from there. Will share if he comes up with something in the near future, if you find something out please do the same

My van is just about 500 miles shy of rolling over 300k. It's had it's own vibration thing going on between 70 and 75 mph the past few months, but unrelated to yours and his issue. Seems more a wheel balance and maybe a couple front things like drag link tie rod end possibly n maybe more. Other than that still rolling along fairly well, love this old van!

 

Intake plenum gasket failing?

My 318 makes the van feel like it has a bad u-joint when one of the cylinders goes down but there isn't the serious rumble you are talking about. In my case it's the rear passenger spark plug which fills up with carbon and bridges the spark gap. I do a 1-cylinder tune-up every year...easier than rebuilding the head.

 

Thank you all. If I ever find out what is causing this and a repair. I'll post it. Still open to anyone who can solve this mystery.