I have a 2009 Yukon Denali that seems to have the same issue. Upon a firm acceleration, there is a "clunk" or sometimes a "clunk clunk" from somewhere in the front. Cant tell if its right side, left side or the middle. It makes a similar noise (may or may not be the same noise) when going over a speed bump (or in my case the end of my driveway). I've tried a hard acceleration with my foot on the brake and there are no noises to report so I don't think the engine mounts are involved. Any additional thoughts or solutions?