throttle body popping noise

If the van is well maintained then you can rule out a tune up. However considering the amount of miles on the motor I would suspect that the cam shaft is worn, (intake valve lobe) and this is cause the motor to back fire through the throttle body. If the car had a miss to it then in would think it was a bad valve. Since it idles well I'am leaning toward the worn cam shaft and not a bad or defective value. This is causing the valve timing to be knocked off everytime the motor accelerates and in turn causing the back firing. Before changing any parts it would be wise to do a compression and vacuum test to confirm my suspicion. The compression test should be normal but I think the vacumm gauge will show a drop in vacumm everytime the worn lobe of the cam shaft goes to operate that value/values. Good luck hope this helps. Backyardmechanic :)