2018 WRX with 78,000 miles. I have had a humming/buzzing noise coming from the rear of the car the past 3-4 thousand miles. Its not the tires because I have changed from summer to winter and back to summer and the sound has not changed. It gets louder or more pronounced as the car accelerates. I have ruled out the exhaust as it stays the same weather i am on the gas or not. Does anyone have any thoughts?