Hi Phil ,
sad to see you have no answers yet . Truth is that I've got the same problem with my Pioneer home DVD system . Although I concede that the surround sound is very good , the system makes an awful buzzing noise after 20 minutes. In medical terms , it sounds like a tinnitus. It stays while the unit is warm/hot , and goes once I leave it for more than an hour to cool down . Then it starts right back up again .
Sad to say I googled "pioneer buzz noise " , and found literally dozens of people who have the same problem on their laptop DVD-Rs .
Simple answer , (I;m no expert ) is that all Pioneer DVDs have a higher risk of this kind of problem . My only solutions are to ask an electrician with more knowledge or just fork out another £300 and start again .
Now I know how common the problem , I won't ever buy Pioneer again . Its been an awful 3 years .
Dr Kush, August 2009