2003 Windstar - After having a very full tank of gas, I noticed a "sweeping" sound (like a broom across a floor)when I went up over a steep grade. Also a small miss or backfire noise at the same time. It only happened on the grade - no where else.
Thought it might be a vibration - checked under neath and replaced worn rubber spacers that hold up the exhaust system. It did not really make a difference. Also undid the gas cap in case that was causing some thing to happen - no difference.
As the tank of gas has become less full the noise is not as noticeable, though still happens. It is usually cold (30 -50 degrees), does not happen any other time. Today when I heard it - it reminded me of a sneeze. Overall van runs excellent. Any ideas on what might be causing this type noise would be appreciated.

 

My van never sneezed. I am glad for that because I don't have a handkerchief large enough!
From what you are telling that the sound comes from the back. See if the spare wheel is mounted right. Look for any signs of scraping under the rear of the car, bumper cover and such. Check the tank straps. Look if the jack is properly mounted, and so on. And if you can't find it wait until it gets worse.
Of course none of that would explain why the misfire occurs at the same time. Maybe something is loose related to the fuel pump.

 

The "very full" tank of gas could either be part of the problem or maybe causing it.
The general recommendation for filling the fuel tank is to stop when the station pump nozzle shuts off. When it is topped to the hilt it places liquid gasoline in places where it isn't supposed to go like the vapor canister requiring it to be replaced because it contributes to running and performance problems.