There is a popping noise coming from underneath?

1993 Pontiac Bonneville 3.8 mileage: 125,000. Our Pontiac is taking longer than usual to go into Drive from Neutral and it will lunge when it finally does so. Additionally, if you leave the car in park and race the engine, you can hear a popping noise from underneath the vehicle. My husband who thinks that it may be the transmission has checked all fluids and belts. What do you think?