There is a strange rattling/squeaking noise inside the trunk, driver's side, above the tire

I drive 45min and 35 miles in the morning and there's no noise. Then after having the car parked for 9-10 hours, on my way back home, starts this very strange noise in the trunk, driver's side, above the tire, and it gets louder progressively. You can hear it from the inside rather than from the outside. It would sound even at full stop (in neutral or parking) and it will only stop if I turn the engine off. I'm really concern now if it's something really bad. I made an appointment at the workshop next week, but I'm afraid they won't find what the problem is. The first few days, it sounded like if a liquid were boiling, not constant and not sistematic; now, it sounds more mechanical, constant and sistematic. I'm not sure if this would have anything to do, but the previous owner used 95-octane gasoline for five years and I started using 91 octanes since 2 full tanks ago; coincidentally, the noise started during the second use of 91-octane gasoline; I did this because I found an online manual that said that 87 octanes and up was OK and the engine seems to be working just fine. I'd really appreciate any help or hints.