Increasing hiss in front right speaker

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.

Any ideas what could be wrong?
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.