Agree with Marakanetz. If you can only hear it if you hold your ear up to the speaker, then it doesn't really matter.
Off hand, I would try switching the preamp feed so that the center channel amplifier input is coming from a different output of the preamp. If it switches speakers, then perhaps it's a noisy channel from the preamp. If it doesn't change, perhaps its the amplifier center channel.
But honestly, what does it matter? You can't hear the noise except in a situation that is not normal (ie. you do not commonly listen with your ear to the speaker).
For what it's worth, some gear is just a bit noisy and I could hear some noise with high efficiency speakers on quite a few amplifiers if I held my ear up. I just ignored it.
Off hand, I would try switching the preamp feed so that the center channel amplifier input is coming from a different output of the preamp. If it switches speakers, then perhaps it's a noisy channel from the preamp. If it doesn't change, perhaps its the amplifier center channel.
But honestly, what does it matter? You can't hear the noise except in a situation that is not normal (ie. you do not commonly listen with your ear to the speaker).
For what it's worth, some gear is just a bit noisy and I could hear some noise with high efficiency speakers on quite a few amplifiers if I held my ear up. I just ignored it.