I have a Harmon Kardon DPR 2005 that I just hooked up to my Yamaha surround speakers. I ran the test to make sure the speakers are all connected correctly, and I set the amp accordingly to match the 6ohms impedance for the speaker set. Sound is great but after 20 mins or so the front right speaker clearly has a white noise sound that slowly but steadily increases in volume. The speaker also emits the audio from the source but also this white noise.
It wouldn`t hurt to try this, turn it off then take the cover off, let it warm up again(1/2 hr.) with a pencil tap the tubes, if any are microphonic or make a noise through the speaker that tube could be the problem.