If the noise becomes more pronounced when you let off the gas, it could be a sign of worn wheel bearings. They tend to get noisier as the load on them decreases.
The sound could also be stemming from the transmission or differential, especially if it sounds more like grinding. Low fluid levels or worn gears can produce similar noises.
Sometimes, uneven tire wear or a faulty tire can create a whirring noise. Make sure to check your tires for any damage or irregular wear patterns.
If the sound is more noticeable after slowing down, worn brake pads or other brake components could be causing the issue.
Look for loose parts or shields that may be vibrating at higher speeds.