That sounds suspiciously like one of those classic urban myths
I don't think it is, I recall reading it on an forum with many ex-Vulcan crew contributing, they seemed pretty sure it was for real. But I can't find the link....
My favourite one really is a true story taken from the incredible book by Brian Shul called Sled Driver;
http://www.fromtheinside.us/thinking/Groundspeed_Check.htm
Ah yes, now that really is a great book.
Ever since I bought it, I've always called the SR-71 "the Sled". It's by far the best name for it.