ID for a rainforest call (1 Viewer)

Hi. I spend a fair bit of time in and around BRVT province. On a number of occasions I've heard a distinct call in the mountains near Long Hai (former Minh Dam base) but never caught a glimpse of the animal/s. The area is wellr regenerated rainforest (post war) with patched of drier scrub.

The call is a strong, clear, deep descending 'hoo doo doo doo doo'. Once I have heard a reply but usually on its own. I am guessing a pigeon or hornbill of some sort? Any assistance in clearing up my mystery sound(!) greatly appreciated!

Apologies to anyone looking at this, i've just identified it from a fantastic VN bird site. For those interested in what i was trying to describe: http://www.birdwatchingvietnam.net/index.html?birdid=358 !

Enjoy