My truck is a retired farm truck and she has some issues when I'm driving over 60 mph or when she runs under 20. The issue at 60 or above is she starts to shut down, almost like how manuals can engine brake. I've heard that it could be a weak fuel pump or plugged fuel filter. The under 20 rattle I'm worried about feels like it's either just under the cab or close to the cab in the engine bay. It's a fairly heavy sound, and I can definitely feel it where my feet are resting close to the pedals. She runs okay other than that, has a bit of a rough start up, but she definitely has power. She has a 460, new aluminum radiator, newish battery and new brakes. I just had a new fuel filter installed and I'm pretty sure they fixed the leak in the hose above the tank. On top of any information you guys could provide for me about these issues, is there anything else advice-wise you could give me to make her run better?
Thanks in advance.

 

I figured out that the under 20 was a loose muffler and the over 60 could have been a bad carburetor tuning, as the floats were hanging almost two inches down. I rebuilt the carb and replaced the gaskets on everything, it's now able to start up without much issue and she sounds much better. I don't know if the high speed issue is fixed as I'm afraid to take her on the highway with out the gas gauge working, but I'm looking into it.

 

Thanks for the update, keep us posted..