21st
The comment was made by Bell following a press inquiry as to why one V-22 had to abort and the back-up used to complete the transatlantic trip to an air show quite a few years back. So there were very few hours on the engines back then and this was directed at Greg's comment that Bell's statement was 'taken out of context', but thanks for the interest.
So, from what I can derive from comments here and Bell regarding compressor stalls, they are normal and I, nor anyone flying accross North America should not be concerned when they hear them while flying(?!). Actually, since they are normal, passengers sould therefore expect to hear them rather than not, since that would be abnormal.
Interesting.