I'm sorry the link just won't work. The video plays fine, but for some reason the links aren't functional.
Yes, I am quite familiar with heart pigs. I'd say if you're familiar with the noise and identified it as a hooting sound, even if you only heard the sound once, I would still schedule a vet visit. Not all heart pigs hoot and they also don't hoot all the time, but I've never had a pig hoot that didn't turn out to be a heart pig.
Age can have an influence on heart conditions, of course, but there are also congenital heart problems that they are born with. The worst of my heart pigs first hooted at about 5 or 6 months of age. He was also a very pushy boy when young, which could just be personality, but I also put it down to him not feeling well and potentially him recognizing he was somehow vulnerable and needed to be more aggressive as a result. After getting on heart meds he was able to be reintroduced to his father and brothers and has lived happily with them ever since. You're piggie may calm down some as well if he does have an underlying ailment that gets treated. Who knows. It's possible.
Yes Oscar is only 18weeks old so he is still little; he is also very bulchy so there is a chance this is the reason why as opposed to hormones.
Anyway, regarding the plopping down after running around.... hard to say. Some pigs do just plop down and chill out after playing or chasing one another, etc.
Does he breathe hard even after resting for a few moments? Does he breathe hard even when he has not exerted himself recently? Do his eye fall shut or partially shut when he plops down? Does he really seem worn out? How about when he sleeps - have you seen him sleep with his eyes closed a lot? Have you witnessed rapid eye movement and him jerking in his sleep similar to how dogs do when they dream? Are you able to approach him when he's sleeping without him realizing you're getting close?
His breathing is fine, he does sleep with his eyes closed but he doesn't have any of the other issues you mentioned. I put the slumping down to them trying to determine dominance as it's usually after he has chased Alan into his hidey!