sharp and piercing high pitch /frequency coming out from Harbeth C7

You need to try a different amplifier to try and locate the cause of the problem/verify that it's really a fault in the speaker.  The standard process of elimination.

what frequency

I agree that you should try a different amplifier with the speakers, and also perhaps a different CD player, though I think the odds are better with the amp.

I had a CJ preamp some years ago that ended up with a bad capacitor in the power supply that generated noise beyond my range of hearing. It was only when my son visited from college and pointed out the issue that I became aware and was able to fix it. 

There is also the possibility that your amp is running out of power when you hear the distortion, but you really didn't say how loud you listen, or in how big of a room.

Lots of possibilities, but you just have to work through all of the components one at a time to find the cause.

I just had similar and, after trying a different amp, the high pitch ringing went away. I just sent the amp back to the manufacturer to check for problems. 

I experienced similar issues when I made the change from 40.1's to 40.2 Anni's.  Changing the amplifier may help some, but the fix for me was experimenting with placement, most notably toe'ing in the speakers so the tweeters align to cross in front of my listening position, as this minimizes room reflections.  I'm also increasing my room treatments with emphasis on addressing first reflection points and increasing diffusion.