I notice the same thing once in a while at Interstate speeds when I accelerate slightly with my 91 4.0. But if I back off a bit, the rattle will go away. (To describe the noise, it's rapid, in time with the engine rpm and might be similar in sound to drumming a glass Coke or Pepsi bottle with a fork. Not a heavy noise like a lifter tick.) In my case, I'm pretty convinced that it's spark knock/pre-detonation. I usually stick with mid grade gas with no ill effects. But,when I notice it happening, I figure out which station I got gas from the last time and get my next fill up at a different station. Sometimes too I'll get half a tank of premium just to make sure it goes away. I think usually the octane numbers may not be quite on the money at some stations (from time to time), which makes the spark knock occur. But it can also be brought on by hot carbon deposits in the combustion chamber igniting the fuel mixture before it should be. It can also be caused by a fuel delivery problem such as low fuel pressure or low fuel flow rate. You can check some of these things out to see if that's what is causing your rattling.