I have had a lot of work on my 2007 Volvo S40 done over the last couple of years and I have been trying to get it to last another year or so before I sell it. Over the last week or so I have noticed a noise while decelerating towards traffic lights. Normally at around 20mph give or take a bit. It sounds like I am going over one of those rumble strips on the highway. It lasts about 1.5 seconds and then stops. My first thought was that it sounded like a fart (I know it's gross) if that helps. This evening I also noticed it when I was putting on the air conditioner at about 35 mph. As the engine kicked in to handle the extra load from the aircon I noticed the noise. It was fainter but that was probably because of the extra road noise.

I also noticed what sounds like a hole in the exhaust when the car is cold and I rev it in park. I am not sure if this is related.

There doesn't appear to be any vibrations.

I had the car serviced about 3 weeks back and it just passed smog.

Here is the sound. It's a bit feint but starts after a second of the video or so. It's much louder in the car.

 

I think its just gas.