I have a 2006 4X4 Dakota SLT Club Cab 4.7L 5 speed autotrans (545RFE). When I reach 40 MPH and the transmission drops into 5th gear and the torque converter locks up I get this humming noise that seems to come from under the left rear seat. When I let off the gas the noise disappears. When I push the gas peddle the noise returns. I am not sure what this is. If I put the tow/haul on, the sound is not present. This is my forth dakota and any driveline problems where easy to pin point. The noises would always come from either the rear or the from front where the transmission is located.
I have been unable to find any serious complaints about this transmission. Could it be the fuel pump making that noise or the torque converter? This is my first 4x4 and there is a lot more stuff under the truck.

Any help from an expert would be great. I am taking back to the dealer, and I have learned if you tell them what's wrong they will usually fix it the first time.

Otherwise its a nice truck.

 

If i am right and i think i know what your talking about my truck does that and i think all vehicle's do that its something with the tranny and the torque converter or something along those line's when you reach that 40 mile a hour mark that's when the i think the converter does that disengaging thing, i don't know if i am even close to being right, maybe i am just talking smack, i don't know so if there's somebody out there that know's what this is for sure then be a expert and tell us what this is lol.