I notice this just from feeling the truck at idle. I don't see the tach guage bouncing around or anything but I can feel the truck rumbling through the seat etc. My passengers have noticed this as well. There doesn't seem to be any discerable problem with the truck other than this. It has been like this for probably 25K miles or so.

I've only had 1 plug and coil replaced by Ford since purchasing the truck new in 05 and that was because the plug went out.

Anyone have any ideas of what I can check for? I'd hate to take it to the dealership and have them charge me for a whole slew of diagnosic and guess work.

Thanks

 

Time to change all the plugs.