pnichols wrote:
I wonder it it's the infamous E350/E450 serpentine belt "rattle"?
This has been discussed a lot in the Ford owners forums. I believe the solution has something to do with an additional, or different, idler pulley between installed at a certain place in the belt's path so as to better support the belt at certain engine speeds.
If your "rattle" is from the pulley referenced above,it occurs at ~ 2100 rpm only. This is due to belt vibration at that RPM. A new belt will stop the noise (sounds like ignition ping) for a few thousand miles, but it comes back.
Here is a link if you want to read more.
https://www.jaycoowners.com/forums/f5/e-450-engine-rattle-serpentine-belt-82252.html#post921181
