Mechanic cannot find cause for sound

I just purchased a used van (2012 GMC Upfitter Savana AWD Exporer high top conversion) has 24,000 miles, drives straight. Carfax shows one owner no signs of accident. Issue is: I hear a creaking sound (like a wooden floor creak) when I heavy accelerate going in reverse. No sound is heard at any other time and unless you floor it in reverse the sound is not there. The sound is coming from the front (I feel certain). Dealership (that sold it to me) had on rack and could not find any issues. I live eight hours away and they gave me thirty days to find an issue and authorized $500.00 to fix it. Problem is a local shop cannot find anything either. Help please!