Nissan Repair: 2006 Nissan Quest, nissan quest, down shift

Question
I have a 3.5 liter with the 4 speed transmission. At approximately. 2000 rpm, under acceleration , the vehicle makes a distinct rumble type sound. It is only under acceleration. The transmission was just rebuilt at 70,000 miles. This was happening before the rebuild, and I thought that might cure it (this was not the cause of the rebuild, the transmission completely failed).The vehicle seems fine otherwise. no shakes or vibration. It almost sounds like it is too high of a gear, but once I accelerate past the 2000rpm mark,or decelerate under 2000  rpm, it is fine,

Answer
Cosmo,

It could be the down shift is not occuring properly.  That is, the vehicle is under load in 4th gear.  If the downshift is not set properly the vehicle will rumble just before it downshifts.  Basically, the transmission needs to be adjusted to downshift sooner.