Some 1999 F-series trucks with 7.3L diesel engines and 4R100 automatic transmissions develop a "rattling" noise in the converter housing area when the torque converter clutch is not engaged may occur on some vehicles. If you place the transmission in drive, hold the brake and raise the engine RPM's over 1000 and the noise disappears the torque converter is likely the cause of the noise. Replacing it is the only effective repair.
Diesel trucks with manual transmissions that have a chattering noise and erratic idle in neutral may have a worn or damaged dual mass flywheel. A new flywheel and clutch would be in order there. Of course other clutch and transmission concerns may cause noise as well.
Adding specific information about your truck to your signature will help us identify problems and offer advice.
Last edited by FordDoctor; 04-01-2012 at 06:17 AM.