1981 F-150, 302/C6. No computer, just DSII ignition. The transmission does have a pan gasket leak and has ran low on occasion, but I keep the fluid topped off as best I can.

I did quite a bit of highway driving today (at least 90 miles), mostly around 55-65MPH, which with my C6 and 2.75 rearend gears puts me around 2500RPM.

I didn't hear the sound til I got near the 70-80 mile mark (out of 110 total today). It sounded like a clattering noise, like a tick-tick-tick sort of clattering. I only heard it in gear when I raised the engine RPM to accelerate. I also heard the sound if I held the brake while I had it in gear and raised the RPM. In Park, however, I raised the RPM and heard nothing.

In addition to the clattering noise, I noticed that I had to rev the engine a bit to get it to go into Drive, and when it did, it would make a squealing noise as it went into gear. At one point, it made a constant, somewhat faint squealing noise any time I had it in Drive. Now and then it felt as though the truck was holding back, like it took more to get it up to speed. I suspect part of this is due to low fluid level, since when I checked it this evening it was about half a quart low.

Now here's what's weird. All the noise and weird behavior stops when the truck cools off. Around town, I get none of this stuff no matter how hot it gets.

Anyone have a clue as to what all this means?

 

The rattling you describe sounds a lot like spark knock to me. Check your timing first, then look for intake leaks, and check the radiator for proper coolant level. The timing spec ought to be on the calibration sticker on the rediator support.
It also sounds like your tranny is slipping or is low on oil. Get the leak fixed soon.
Good luck.

 

That was actually one of the first things I thought of, spark knock.

I could use a new carb gasket, since mine is pretty old. I'll also check for other leaks as you suggest.

To my knowledge the radiator is at the proper level, since there was still some coolant in the overflow tank. I will check tomorrow though and let you know for sure.

I'm not sure when I can get the leak fixed, since it's almost $140 to get it done at a shop here...I'll have to wait til I can get a relative down here to help me fix it.

Thanks