Using VIN at 9v makes high pitch sound

I am splitting 9v from wall transformer to VIN and I get a high pitch sound coming from arduino, doesn't sound right. Any idea what could be causing this? Multimeter tells me I am definitely getting 9v.

Dosn't sound right to me either.
What is the arduino connected to? Are you sure it is the arduino?
Are you sure that it is DC comming from your wall supply and not AC?

Perhaps a SMPS 9V supply with poor filtering is causing a lot of ripple current. Some larger ceramic bypass capacitors are piezoelectric, you may be hearing that.

What else is connected to the Arduino? Shields? Inductors with loose windings can cause noise, too.

Grumpy_Mike:
Dosn't sound right to me either.
What is the arduino connected to? Are you sure it is the arduino?
Are you sure that it is DC comming from your wall supply and not AC?

its definitely DC. arduino isn't connected to anything, I disconnected everything to isolate it and make sure I wasn't doing something silly.

Can you tell where on the Arduino it's coming from? Sounds like a SMPS power supply to me too. What's the power plug like? Big heavy block, or one of those little, light-weight ones? What's the current output from it?

Pictures are worth a thousand words. Maybe a photo of the PS showing the label would help.

Also, make sure the output of the PS going into the Arduino is the right polarity.