High Pitch Sound From 12V Fan PWM

Hey,

I recently took a 12V 0.2A 92mm fan out of my broken DVD player. i tested it and it was absoultly fine but it was a bit too powerful so i decided to build a 555 astable circuit for PWM. (R1=10K,R2=10K,C1=0.1UF). The output was connected to a BC547C Transistor which was sinking the neutral of the fan. the positive was connected to 12v. However when i turned it on i got a High Pitched tone coming from the fan so i imediatly turned it off. i then got my multimeter and measured the frequency to be approximatly 460hz and the duty cycle was approximatly 64%. why is this noise being produced and how can i stop it

You just builT an audio generator. The fan is magneto-strictive, meaning that works as a loudspeaker

This is an application where you should have used a "linear" regulator, not PWM. Also, since most of these "muffin" fans have very complex internal electronics (hall-effect sensors, HBridges, speed regulator) they are not ameanable to being speed controlled by varying their voltage. Down load the data sheet for the specific fan you are using and study it...

Thanks for the reply. I looked at the fan and found the model number to be "AFB0612MB". i googled this and came up with this website: Fan Website and the datasheet: Datasheet. is it possible to change the fan speed with an LM317 and varying the output voltage with pwm