That loud rumbling noise under my Infiniti is driving me crazy

[orwinkeal]

Hi everyone, I need to ask if anyone else has dealt with this because I am starting to lose my mind over a noise issue. I drive an Infiniti M37 and for the past two months there has been this deep rumbling sound coming from underneath the car. It is worst when I first start driving in the morning but it never fully goes away. At first I thought it was just road noise or maybe my tires were worn out. I replaced all four tires and got the wheel alignment done. The noise did not change at all. Then I thought maybe it was a wheel bearing so I had a shop check that. They said the bearings were fine. I am not a mechanic but I have been trying to figure this out by listening carefully. The noise gets louder when I accelerate and seems to come from the middle or rear of the car. Sometimes I also hear a faint rattling sound like a loose heat shield when I go over bumps. A friend of mine who knows a bit about cars said it might be something with the exhaust system like a broken hanger or a hole in a muffler. That actually makes sense because the car feels fine otherwise no shaking or warning lights. I found a shop online that advertises complete exhaust system repair for Infiniti and I am thinking of taking it there for a proper inspection. But before I do I wanted to ask if any of you have had exhaust problems on your Infiniti that sounded similar to what I am describing. Was it a rusted pipe a broken clamp or maybe the flex pipe that connects to the engine? Also how much did it cost you roughly to get it fixed? I do not want to walk into a shop completely clueless and end up paying for a whole new exhaust when maybe only one small part needs welding or replacing. Another thing I am worried about is if a bad exhaust can affect fuel mileage or engine performance because I have noticed my gas consumption seems higher lately but I am not 100 percent sure. I would really appreciate any stories or tips from people who have been through this with their Infiniti. Thanks a lot for reading and for any help you can offer.