Airtunes interference - terrible ringing noise
Two things to consider:
o The power source, and
o Wi-Fi interference
On the former, have you tried plugging the AirPort Express Base Station (AX) in either another outlet or to a noise-canceling power strip?
On the latter, I suggest you perform a simple site survey, using utilities like MacStumbler or iStumbler to determine potential areas of interference, and then, try to either eliminate or significantly reduce them where possible.