Your car might be making a humming noise for a variety of different reasons. For example, there might be something wrong with the transmission. If you are driving a manual, there might be something wrong with your gearbox.
It could be a loose belt-driven accessory like the alternator. When accessories aren't firmly bolted down they can get out of alignment from the belt causing squeeksband rattles that are proportional to engine speed rather than car speed.
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