No not the spirax, I'm familiar with that on units and on locos, and that's more a clicking than a twanging, if you see what I mean! You wouldn't get that on a mk3 sleeper either I don't think.
My sound is a bit like pan pipes I suppose, but metallic rather than reedy, and it's continuous when it's happening. The pitch varies slowly, maybe rising and falling every three or four seconds. It isn't related to heavy braking like the din you get on mk2s but could be connected with application of the brakes in that I've only heard it either when at a complete stand or moving at speeds of 10mph or below.
I'll try and get a recording the next time I hear it.
On the sleeper mkiii I was on last night there was a terrible noise on braking. It was like someone playing a violin. Awful!
( it could have been suspension related but only occured when the brakes were applied)
As many times as I've been on a mk3 sleeper I've never got noise from the brakes other than when a mk2 has been next to my coach and I've been able to hear the racket from that distance. You can get a ghostly rising and falling tone which I've put down to the 67 as it takes power, you don't hear it with an electric loco up front. But that's more like a heavenly choir than an old violin! It sounds like there was something wrong with your coach!