2.8s tend to do this.....sort of a pop popata pop as you let off the gas between gears. It does not seem to hurt anything, and may be a combination of incomplete burning of fuel and timing and perhaps temperature. Injector cleaner added to the fuel may help.
edit: trying to remember what a water balloon popping sounded like....maybe you have broken a motor mount.
[This message has been edited by Gall757 (edited 04-26-2014).]