Ok so my mare makes a noise when in the upper paces, certainly at a gallop. she has a very muffled neigh, think Marge Simpson on too many fags!
I've never found it to affect her performance, Although it can be a little bit worrying for others, I just reassure them she's fine. I suppose it depends, what job you need your horse to do. I've done endurance rides with her and she's always been more than up to the challenge.
I have spotted a horse for sale which has this listed as a reason to reduce his price. I have had little experience this noise myslef, people have sometimes told me its because my horses sheath needs cleaning but its always kept clean. Like my horses to be able to do a bit of everything xc, sj, odd dressage test, hacking, odd show, bit of everything.
I am not sure its just a phrase I havent really come across on an advert for a horse I like the look of. Which said 'Can slightly hear wind in extended canter hence very low price.'