Voltage might be too low to run motor or too low for motor to reach high enough speeds. For example 2500rpm Ty-143 at 5v is 1165rom, Gentle Typhoon 3000rpm at 5v is 1251rpm. On PWM control my TY-143 idle down to 550-600rpm .. that's slow enough for me.Is there anything wrong with replacing the 12v wire on a pwm fan with a 5v wire? I'm doing this to a spare 4 pin extension wire atm because I want to reduce the speed/minimum speed of my pwm intake fans.
You seem having wrong understanding of the word CLEANER ... such low end buck converter will backfire electrical noise and the pollution it might cause instability at other circuits.but a buck converter is much cleaner(can just run off the 12v your pwm fan already has) and can just be electric taped to your pwm wire.
You may combine L7809 and L7808 and L7806 in a single PCB and use a miniature switch (three positions).Yeah ok that looks 'cleaner' and safer than the buck converter, no variable output voltage though.