The sound you may be hearing could be from a collapsed trachea...this is from people cranking their horses heads back...totally "OVERchecking" them to get the desired head set. My big soap box...this makes me crazy mad.
People just don't give the driving horses time to develope either...they RUSH, RUSH, RUSH...not giving the horse time to mature physically before asking them to be the big shining star...ahhhh, soap box again!
Here's the form follows function deal...some horses aren't made to have head sets like this. Ok, another soap box of mine...but, I'll save that for another post...but, the greed of some people don't give a flying flip...at the expense of the horse.
Most likely why the horse is not doing at roadster and, does it in pleasure is because the horse is more collected up in pleasure...they may be allowing the horse to nose out for the roadster thus he is getting the air he needs...this is what I am putting together here.
I know there are many other driving people out there that have been in a class and, you can hear it coming a mile away. If
I was the judge...I would not place that horse for anything. The horse can't get the proper amount of air...I cannot imagine what the horse is going through...although I have a good guess as, I almost drowned as a child...no air...is not pretty.
Anyway...IMO I would have the horse scoped...that way you know for sure whats up...there are other tests you can do as well...that a good Vet will guide you through I am sure.
It could be a something else totally...but, the Vet is the first place I would head.
good luck & hope this helps
Lis
p.s NO it is not natural older horse to be making these sounds.