Squealing Unico

I have had a Unison Research Unico amp (hybrid) for about three years. Just recently I noticed a squeal coming from the left channel. It basically sounds like a distant tea kettle whistling. The sound occurs no matter what input I'm using (CD, phono, AV). Recently, the squeal has also come out of the right channel, though not as loud. It's not a very loud sound and is not always present. Of course, I would like to solve this problem. I've done a little research and will probably just swap out the ECC82's and see if that changes anything. (I have not changed the tubes since I got the amp.) So, I turn to the collective wisdom of the Audiogon Community: any ideas?