I have a 97 F-150 8 cyl, automatic. When I first start driving and the truck is switching between gears, it makes a rumbling sound for about 2 seconds. I think it happens when It's shifting from 3rd to 4th gear. What could this be? I hope it's not the tranny!

 

That is possibly what we call torque converter shudder. You can try flushing the transmission to see if it will help the problem. You can also buy an additive that works really well. Ask for anti-shudder transmission additive. Cheap way to see if that is the problem.