1990 Dodge Ram window van. Audible vibration noise from 60-70mpg

Check the wheel bearing. If over or under torqued, it may give problems. Check the drive shaft is not bent from hitting a large stone whilst moving.

you may just have bad u-joints they can cause vibration when driving you can or may hear a clunking sound when you put it in gear.

sounds like your lifters

Your noise sounds like it is power train related,If you have eliminated the engine,Since it doesn't make the noise while moving and in neutral.
It sounds like a universal joint in the drive shaft.Do you notice a vibration
along with the noise?

First get your wheel balance checked .... Sometimes the balance weights come off !
If that checks out ok and the tires are good then I would be checking the rear wheel bearings.
Let me know how you get on.

Noise is coming from a dry u-joint and or the carrier bearing if equiped. Usually a u-joint "cherps" in reverse first then only gets worse with time. If it starts vibrating at highway speeds replace them immediately.

Check fuel pump pressure. sounds silly but my 1500 ram van did this 2 months . replaced a lot of parts then left me sitting on side the road. Cause was fuel filter and pump.

Bad hub bearing. Drive the car on a straght EMPTY road .@30 mph or when noise is audible, **** the wheel left than right (similar to a race car warming. tires) and listen if there is a change in the sound if so its a front bearing if not it should be a rear bearing .Jack the rear of the vehicle up and spin the tire holding onto the strut. Feel and listen for a vibration or noise. This should pinpoint that elusive noise.Let me know Good hunting

I have a 2002 Olds van and it also makes a noise under the front driver wheel well. Mine was mostly when I hit potholes or large dips causing the shocks to compress. The problem is a metal to metal contact between a bushing lip and the suspension. On this side, the bushing is manufactured opposite of the passenger side. The lip is up and causes the two metal pieces to rub. I had to grind down the lip so it would be like the other side. All is better now. I'm not sure if this is where your noise is coming from since it gets louder with speed. I would assume yours is a bearing. Sorry.