Car makes like a blowback sound. Could this mean anything serious?

What is your question?

I've been noticing for the past week or so that whenever I go from 0 mph to about 40 or 50 mph with my car, At about 4,000 RPM's the car does a blow back type of sound. It sounds like a popping noise but not so loud just noticeable. Would someone be able to tell me if this is a good thing or bad thing?