Not noise in the usual sense, but radio noise. A system I supplied about 6 months ago has worked perfectly (as mine all do!) and was still apparently innocent of any sin until a radio ham next door complained about excessive radio interference between 7mhz and 28mhz, and had triangulated the source to my machine. I fitted a replacement PSU and the noise disappeared at once - and I then tested it by connecting a PSU tester to it and reconnecting it right next to the short wave receiver. Fault confirmed. The whinger then pointed out that his wife was a JP and clearly expected me to run away - not so because I then asked him to pay for the repair on the grounds that his radio equipment might be faulty. We agreed that I would pass it to the local Trading Standards office for them to check if it conforms to permitted emissions. We will see!