Have a 2006 Ford Expedition. I had noticed about 6 months ago that every once in a while it would rumble or shake when I was going 45-55 mph and with my foot only lightly on accelerator. Now it happens a lot (not all the time) when I have light pressure on the accelerator and pretty much any speed over 40 mph. Note that if I floor the car or have heavy pressure on the accelerator it runs great!
At first I thought this was a first sign of transmission failing but it doesn't spin and during my last inspection the shop ran diags and there were ZERO errors from the transmission. Again I have had 5 cars have their transmission go out and I can tell almost instantly what is going on. This never "spins" or loses contact. And now that this happens every once in a while past the last gear I think I have ruled it out. What do you think the cause is? (Mechanic takes it for drive and says it runs perfect)