2003 Pontiac Montana humming vibration

Approx 1 1/2 years ago we had the transmission rebuilt in our mini van. Last October we had some hub assembly work done. The next day we felt a slight vibration and humming sound when the van reached 40 mph. We took the van back to the shop and they couldn't find anything wrong - suggesting we have the transmission checked. We did, and they found nothing wrong. The problem continued but gradually lasting longer - eventually it would begin around 35 mph and last until 50 mph. This week it became much worse - beginning at 35 mph and continuing at 70 mph. We had a different person check it out and he couldn't figure it out. So we returned to the shop that replaced the hub assembly. Again they checked it out - could confirm what we were feeling and hearing, but could find nothing wrong - and suggested we have the transmission looked at. We did this again and the transmission place also confirmed noise, but said transmission checked out fine. SO, we took it to a dealership for a fourth opinion. This company says its in the transmission - probably the sensors. They seemed surprised when I told them the transmission was rebuilt 1 1/2 years ago. Since then my husband has noticed grease and metal shavings around the CV joint. Does this sound like the transmission sensors? It hasn't been that long since the transmission was rebuilt, it shifts fine, and the transmission shop has checked it two times and indicated nothing was wrong. We just do not want to be ripped off if it's not really the transmission and something less expensive.