i have a 99 ranger 3.0, ther is a strange rattling when im on the freeway a try ro accelerate. it only happens when im traveling over 50 mph and it seems like i have less power when i try to pass someone. any thoughts?

 

if it does it in 4th gear as well as 5th gear then i would have to imagine it is driveline related. if engine speed is not a factor, and transmission speed is not a factor, only vehicle speed and when under a load. suspension parts or imbalanced wheels also can do that too though.

 

Something else to check would be the exhaust system. Maybe a heat shield or something.
Or since there seems to be less power, maybe a clogged or messed up cat or muffler?
.... just guesses.

 

yeah harmonics are a strange thing. on my bike one time i had a engine mount bolt that came loose and would only vibrate at 6k rpms.

 

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.