02 Grand Am starts with a pop/bang

My 02 starts with a pop/bang, usually in the mornings but sometimes later in the day after several starts as well. This 'pop/bang' sounds similar to somebody being under the hood and popping a paper bag of air while I start the car. Sometimes it's louder than others. I also have poor fuel consumption...I was down to 180 kms at one point, not much better now. There doesnt seem to be any other symptoms, such as back firing, hesitation when excellerating, stalling or anything else.
Any help with this issue would be greatly appreciated.
Thanks