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This is car is known for two different 'rattling' noises while accelerating. The first and easiest to check for is a loose heat shield on the exhaust
. Make sure the vehicle is cooled off, and smack upwards on the exhaust from underneath the vehicle at various places. If you hear a rattle, you'll need to investigate further. These heat shiels can usually be tack welded, or tightened up using hose clamps.
The other possibility is what's known as pinging. It's possible you have a dirty Mass Air Flow sensor, vacuum leak, restricted fuel filter, or a failing fuel pump. You can try filling it up with premium unleaded as a higher octane fuel will resist pinging better than a low octane fuel. If it does not ping with premium unleaded, you know the problem is in the fuel system and that warrants further investigation.
Depending on your year and engine
, there may be a computer programming update available through a Ford dealer that also may help to correct a pinging problem. This should only be done as a last resort if diagnostics don't reveal any other problems.