I'm considering buying a used 1999 express 3500 with just under 100k miles. I test drove it and everything was fine except the van started to vibrate loudly above 45 mph. It was more like a low pitch rumble then a rattle. It seemed as if the vibration was coming from the rear, the steering wheel was unaffected. Engine speed didn't seem to have an effect, revving it at idle didn't create any vibration and letting off the gas on the highway doesn't stop the vibration. The owner of the van said he kept a lot of weight in cleaning equipment in the back and the vibration didn't start until he removed the equipment. He thinks it's a bearing issue? Anybody have any idea what this could be? Should this be a major deterrent for the van?

 

I'd check the universal joints, and the bushings on the transmission and the differential, where the splined piece enters.

 

wheel bearing typically do not cause vibrations.
+1 for driveshaft u-joints and center bearing if its a two piece driveshaft.

 

Tires

 

That would change with speed, wouldn't it?

 

it would have to be the back tires as the vibration was not transferred into the steering wheel

 

Does this Van you speak of have a Carrier Bearing .....
that supports the drive shaft....
take a look under the van from the transmission to the
rear diff... if it has a carrier bearing.. THOSE are very
easy to over look in the cause of vibration.....