I had that same problem every once in a while on opiates.
However, I pretty much figured it out.
When I was on opiates, I was snorting around 10 mg oxymorphone throughout the day. This lasted about 8 months.
Anyways, the hiccups came on about 1 month in.
What was the source that I found for these hiccups?
Stress.
Yes, stress.
Once the hiccups started to really piss me off I researched all the "causes" of hiccups, and after eliminating almost all dietary and genetic reasons, I was left with the most likely reason being stress.
I thought about it longer, of course over a period of about a month, and realized that I got pissed really fucking easily while on opana.
What I started doing was taking 1 mg xanax a day along with my opana.
This fixed the problem. Probably because I was too zooted to think about hiccups (or anything stressful, for that matter).
Obviously, I don't recommend this.
Anyways, I quit opiates a while ago and this problem went away. I came to realize that I was subject to somewhat of a opiate-induced anger, which is apparently fairly common. A better way to describe it would be a "decreased threshold of irritability."
Just some food for thought.
Good luck.