when driving or sitting still with the blower motor on high there is a roaring sound seems to be caused by air flowing in to the car and flowing out. The faster you drive the more the sound increases with air being forced into the car…
2016 Mazda Miata MX-5 - Noisey blower
You are referring to the heater’s blower right? If so there may be an obstruction in the air path somewhere. The incoming air goes through those vents under the windshield and often those also allow small twigs and leave-parts into the air passage, which can result in weird noises. The debris can work its way int so much it interferes with the blower’s fan cage, even louder.