I have a 99 F150, 4.2L V6, rear wheel drive, about 131,000 miles. I just noticed today while driving a low rumbling noise any time I am on the gas. It sounds like an aftermarket exhaust and I can feel the entire vehicle vibrating. All gauges are ok, the check engine light has not come on, I am not noticing any strange smells like something is burning, and overall performance of the truck seem ok.
is yours an automatic transmission?
If "Yes" it might be torque converter shudder.
Hard to say much more without a better handle on the "rumble".
Is it audible? that is to say, could somebody standing on the side of the road here it rumble when you drove by?
If it is audible, is it an exhaust noise, or mechanical noise?
Does it do it in every gear?
Does it stop after you reach the speed you are accelerating to?
Will it occur if the transmission is in neutral?
Any other conditions where it does or does not occur?
We can start here and maybe go forward pending more info.
It is an automatic transmission. I can clearly hear the sound while driving. I definitely think anyone could hear it as I drive by. I think it sounds like some cars I hear with an aftermarket exhaust system that makes a 4 cylinder rice burner sound like a throaty v8. It only seems to make the noise when I am pressing on the accelerator. The faster I am driving the louder the noise is so it happen in every gear. When it is idling the sound is much more faint. I have not tried in neutral yet.
The engine does not hesitate, the rpms do not flutter, nothing else seems to be affected.