Silencing coil whine (?) by blocking fan vent
No. Do not do this.
Power supplies typically push their exhaust out the back of the PC from that particular vent.
The other vent is, depending on the case fan setup, either an air input for the CPU fan or an exhaust for the CPU fan.
The vent you are proposing blocking is the PSU exhaust. Air is drawn into the case by the other fans, and a fan in the PSU itself pushes all that out the back of the PC there. By blocking it you will be preventing the PSU components from shedding heat. This may cause your PSU to overheat and/or die a lot sooner than it otherwise would.