I've got a 1999 XLT scab with the 4.6 and when at a steady rpm it rattles. It kinda sounds like pre-detonation. Also it gets worse the hotter it gets and since I'm in Phoenix, it gets really hot.
it's not horrible but the knocking is annoying.
Any ideas what it is and how to stop it?

 

What is the octane rating on the fuel you are using? Usually mine pings on low grade.... midgrade and premium ir does fine on, however.

 

The OP says that it rattles (detonates) at an even RPM. That doesn't sound like pre-detonation. That description sounds more serious. Running even rpm's shouldn't give any kind of pinging problems. That should only show on acceleration and shutdown (run-on after shutoff).

 

Regular around here is 87 octane and 10 percent ethanol.
It is pinging just ever so slightly at steady rpms but doesn't do it until it is completely warmed up. After driving for 15 minutes or so and it doesn't do it under a load.
does a 4.6 have a knock sensor?

 

Yes it does have a knock sensor.

Ok, why not try a tank of 90 or better fuel and see how it does off that. Make sure that you are 1/4 tank or less, then fill it up and try it out. You may end up having to use that grade of fuel.

I don't know how long you've had this truck, but is it possible that a prior owner used a tuner and set it up for a better fuel/power mix? Just a thought. I'd still try the better fuel and see if that make it run better.

 

Is there a way to tell if someone put a tuner to it?
when I was a kid and I advanced timing too far, I'd get noises like this. I timed my Volkswagen by ear when I had no equipment and if too advanced, it would ping. Same with a 69 Dodge van I had. I retarded the timing a degree or so and they ran perfect. That's what it sounds like to me. I'm assuming there is no way to mess with timing on these other than a tuner. Is that correct?

 

Are you sure it's internal ? Did check the cat heat shields ? All the plugs tight? PCV valve in good shape ?