K8N-E deluxe+USB...faulty mombo or faulty PSU?please help!!!

Hello all,
I am writing from Greece.I am a proud owner of an athlon 64 3000+ and a k8n-e deluxe mombo.I have the following problem;when i plug in my USB Alcatel Speedtouch USB modem,a very high pitched sound comes from the mombo...it is then and only then when this sound is produced.As long as the modem is plugged in and operating the sound is alive...it dies as soon as i plug out the modem.There is nothing else wrong,only this REALLY annoying sound.I have two theories.
1)My PSU is of low quality and that causes the problem (i own 400Watt PSU kinda no name,it's called Pegasus)
2)The mombo is faulty.

Please help me me cause i dont want to go and RMA the mombo if something else is wrong...


PS1:The USB Alcatel Speedtouch is known for it's power problems cause it drains 500ma,but with my previous motherboard and supply (A7N8X deluxe revision 1.04 and PSU 350) nothing bad ever happened.

PS2:I have connected one SATA 120Gb hdd (Western Digital. JD model),1 NEC 3500A DVD-RW and Asus 9600XT graphics card.

PS3:I have tried bioses 1005 and 1006,o/c and non-o/c cpu and mem,nothing works.