When I first heard my engine sound I thought it was on the bottom end, but I was confused why it would come and go. When I took it to the shop and we put it on a lift it was very obvious then that the noise was not coming from the bottom, but on the top side. This led us to question the valve train.
I am not saying what it is, but I am offering my episode as a possible consideration. The fact that it is continuous might indicate lower problems. If the van is on a lift you can hear the bottom end much better.