Unacceptable noise from Seagate Power Adaptor

I bought the 3,5″ Seagate BackupPlus Desktop, 3 TB, USB 3.0 for 144,90 Euros. When I started to use it I heard a very loud high frequency noise coming from the power adaptor of the disk. The weird thing is that this annoying, unacceptable noise only accurred when the disk was in the stand-by/sleep mode. When the disk is in use the noise is gone which means that consuming power the power adaptor doesn’t hum/hiss. I had to give the disk back to the vendor. He was quite unfriendly telling me that every power adaptor makes noise. This is true but this noise was louder than 50 dB (A), so it is far from every industy standard! I changed to a WD.