Lincoln/Mercury Repair: 1994 mercury grand marquis @136600 miles, mercury grand marquis, grand marquis

Question
Hi There, I recently inhereted my grandfathers old faithfull grand marquis,when warm it rattles under acceleration especially when it changes into 3rd & 4th gears, as the rpm increases the rattle decreases.
At all other rpm the engine runs incredibly smooth
Maybe its the carbon build up/ping I have heard about?
Any ideas?

Answer
Hi Chris,

Where to start? Let's try this: Run a couple of tanks of Shell premium through that baby and see if the rattle goes away. If it's a ping, and grand-dad drove like, well grand-dad's do, then there could be some build up on the valves. Shell is my prefered fuel and makes my Navie purr like a kitten. It also has an additive that cleans up the valves and exhaust/intake ports. You should notice a difference about a third of the way through a tank.