Our 2002 Yukon has 74,000 miles. Recently, it's started a worrisome thing. When we accelerate from a full stop, for example at a traffic light, the car seems to make little "chugging" type noises. My husband describes them as almost like little hiccups -- not the sound, but the frequency because they aren't constant and once you get above about 15 mph they stop. We've noticed that this only happens in 4WD, not 2WD,and it seems to be getting more frequent. Accelerating quickly or slowly doesn't seem to matter. We've had our service station look at it 3 times and they can't find anything and say the car is in great shape. What should we do?