Step on gas makes a grinding sound

I have a 2006 dodge caravan with 63000 miles on it and if I drive it before the engine is warm, when I step on the gas it makes like a grinding sound. Any ideas what it may be?

When you step on the gas before the engine is warm it makes a grinding noise. If you let off the gas then the grinding stops. But after the car is warm the grinding stops. But anytime the car has been sitting the same thing continues.