what causes valve rattle?

what is your timing set at...

What you are hearing is most likely pre-ignition or pinging, which is when the fuel/air mixture inside the cylinders ignites without the spark plug firing it off.

Pinging is caused by a number of things:
1.) Hot spots in the combustion chamber such as carbon deposits from high mileage or excessive oil consumption.
2.) Wrong heat range on the plugs.
3.) Too much initial spark advance.
4.) Too low an octane rating for the engine setup.
5.) Lean fuel mixture.
Lean fuel mixture breaks out into several sub categories:
A. Vacuum leaks
B. Air entering the intake without passing through the MAF
C. Failure of the MAF, BAP/MAP (Baro or Manifold Air Pressure, same sensor, different name), ACT (air charge temp), or ECT (engine coolant temp). These should set a code in the computer.
D. Leaking exhaust gases from EGR valve at WOT.
E. Clogged fuel injectors.
F. Fuel injector wiring problems causing injector not to deliver rated flow.
5.) Computer problems: (computer problems are not common like sensor problems)
A. ROM has bad data in fuel or timing table. This should also set a code in the computer.
B. Failure of one or more of the computer's driver transistors for the fuel injectors. No code set on this one.
6.) EGR not opening at cruse power. The computer adds timing advance to compensate for the slower burning speed of the EGR diluted intake charge.

Have you dumped out the codes?
Here's the link to dump the computer codes with only a jumper wire and the check engine light. I’ve used it for years, and it works great.
See http://www.batauto.com/Ford/
OR
See http://www.dalidesign.com/hbook/eectest.html for more descriptive help
OR
See http://www.mustangworks.com/article...c-iv_codes.html