Antifreeze smell when accelerating and it is not the heater core

I drive a 2006 Chevrolet Equinox with 116,000 miles . I have a slight antifreeze smell coming from my heater/air conditioning vents. It is strongest when I first turn on the air conditioning or the heater, and then it for the most part goes away. But when I accelerate it seems to have an even stronger smell again, when my speed levels out or when I am driving at consistent speed for instance 55 mph the smell for the most part goes away. This has been gong on for 6-8 months now, and I have not had to add any antifreeze to my car. I have taken the car in 2 times to 2 different mechanics, and paid 2 times for a diagnosis They have both checked my heater core by using a pressure test of some sort., and they both say that they detect no problems. Yet, there has to be something going on here. My car should not smell like antifreeze. If anyone has any ideas on what it could be or what to do next I would appreciate it. Thanks!