See if these vents and maybe others in the building are connected to a broken exhaust system. There may be a broken, off, or missing fan meant to blow air through a common exhaust vent.
If so ... get it fixed. For many reasons, mainly removal of moisture and smells from your building.
But also, a flow of air away from your vent may reduce the sound levels slightly by masking it in white noise.
If you need to reduce the sounds further without blocking off the vents you could also replace the ductwork between them with sound attenuating ducts. This probably isn't practical if you're in a rental but if you or the owner ever carries out any renovation you can try to get this done.