CDX power supply problems

I bought my CDX cordless. So the guy in the store built a power supply for me. The CDX works... but if makes a funky noise, like a high-pitched humming, when turned on, and that noise also comes out through the composite audio output. I wonder if the polarity is inverted in the power supply. I have nearly no acknowledge in this stuff (I can solder wires, and that's it), and I fear blowing it up by switching the wires.

What I ask is: will my CDX die if I switch the power supply wires? Or there is a change they are actually switched and that's causing the strange humming?
you could just go out and buy a brand new universal powersupply. Thay usually come with a range of differen't conectors and also voltage selection switch. It's a much better option than switch the polarities around. Which in my book is a big NO NO.
Your CDX works, so the polarity is definitely correct. As for the humming, it could be due to insufficient filtering or a short in the power supply. AC, by definition, has a frequency (this is typically 50/60Hz depending on where you are), so any power supply that outputs DC needs to deal with eliminating it.
I couldn't find an universal power supply with the proper amp, but I might check better stores.

Also the humming is different from the usual 50/60Hz humming. It's high pitched and it's not uniform. It's like... lemme see... well, when you're at beach, put your head underwater and you can hear the sand moving... that's the only noise I ever heard that sounds like it.
ExCyber is right - the problem lies in the construction of the power supply. The guy in the store needs to fix his power supply - the noise you are hearing is either because the ampage is too low (causing "ripples" in the supply) or because he has not filtered it correctly (not doing enough about the ripples) or both.