How long does this continue if you don't put your foot on the gas? Try this in a nieghborhood or large parking lot.
Can you coast with your foot lightly on the brake and still hear it?
Does it happen in reverse gear?
If you let the car start coasting in Drive, then move it to Neutral, does it still groan?
Does it happen if you are turning left or right?
Try releasing, setting, and releasing your parking brake to make sure it's not sticking. Try using your parking brake when you are coasting slowly.
1980 450SL @ 103,000 miles
1966 230SL @ Unknown Milage
2006 C230 @ 110,000 miles (Sold)
1994 E420 @ 118,000 miles (Sold)