Automatic coupe 2004. Engine noise recently started sounding deeper, louder especially when I let go of the accelerator. It sounds as if it was down shifting. At first I thought it was the new tires but it is too loud.

Another possibility is that for the most recent oil change I used 5W 40 since I drive 500 miles + per week and I heard here in a forum it is recommended. Before that I only used 0W 30 (I think)

If it is due to the oil I guess it's ok, but I'm concerned it could be something bad.

Any thoughts are appreciated!