I hear a rattle in the engine when in park and I rev the engine and when I am driving and I let off of the gas and slightly mash back down on it but only in D3.When I shift the trans into over drive the noise seems to go away, and I cant hear it when I apply the brake and mash on the gas to put a load on the engine.What could it be?

 

When I was about 16 years old I had a Plymouth Valiant that did the same thing! It was a loose transmission! The bellhousing bolts were loose and the whole transmission was rattling around! After tightening the boltsback up all was happy and peacefull again. I hope that isn't your problem, but you might check it out just to be safe. Have a friend move the tranny from drive to reverse while you look under the hood and make sure thereis no movement at the bellhousing. Another thing to look at would be your U-joints. Set your parking brake and put the tranny in neutral, (this is safest if on flat ground!) and try twisting the driveshaft back and forth. If you hear any noise or feel excessive movement you probably have a bad u-joint.

You might also check the flexplate bolts too! It sounds like what ever is loose stops rattling when under pressure. Hopefully it is just something loose and only needs tightening up!

 

Thanks for the info.