I remember one bright blue-sky day,mid-50`s,out in a field near our house,flying my Mills.75 cc model,and becoming aware of a very strange deep noise; then saw in the distance a contrail.Gradually the noise got louder,contrail got wider and one could count about 8 trails.I knew it was a B-36 as I`d seen pictures in the `Eagle` magazine,probably at about 25-30k,very noisy,and then fading away heading towards N Ireland,then seeing another one coming,more contrails...as one disappeared ,another would come along at about 10-15 minute intervals,for what seemed like hours...the neighbours were all out watching,so I was able to tell everyone they were American B-36s,which probably restored some credibility from being complained about with a `noisy` diesel model aircraft....
Think the noise would be best described as a mix of Shack and Tu-95 Bear...once heard,never forgotten.....
You could throw in a bit of `Huey` into the mix as well....