Sounds a Bit Like BackFire

When I start my 07 Malibu SS at about 1/3 up on the heat scale, and then at about halfway, the car makes a sound which to me sounds like a "muffled" backfire. It seems to occur when the car is being driven
fairly slow. I don't usually warm the car up much: I just wait a minute for things to settle down then go. The sound comes from near the engine, and then from what appears to be towards the muffler. At the same time the engine seems slow down suddenly for a brief moment. The car continues to run in any case, but I was concerned that there might be a bad sensor. Would any of you guys have some ideas?
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Take it to the dealer and have them check it out.
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Start letting ur car warm up a little before you take off.
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