We looked at a similar issue for the (now failed) Mt Coot-tha zipline project. As there are busy roads, a quarry, aircraft noise impacts on the bushland already the ambient noise was high so the impact of the project was minor in terms of standard noise assessment metrics.
I saw a bigger issue being a potential predator response reaction from wildlife with a high speed object audible flying through the tree canopy. Same could be said for mountain bikes I guess but the (project) ecologists reckoned it was a non-issue. The wildlife seem to get used to those kind of sounds over time and move back into those close proximities to trails etc. when comfortable again.
My thing with these environmental conservationists is that, it's a bit 2 faced in what they're trying to do in a lot of bushland around Brisbane but I generally don't have a problem with what they stand for. From my own observations, walkers are just as bad and if not worse than mountain bikers in these Brisbane parks, if they throw us out of parks on conservation values alone they'll need to do the same to walkers and everyone else that visits these places, instead they're trying to use votes from these people and turn them against us. Hardly anything in these parks complies with the conservation act but yet they want to bully Mountain bikers out of there. The few mountain bikers that go building illegally are giving them ammunition, as you can see with all the pics they're posting across the internet and papers.
In the big scheme of things concerning Brisbane parks we are nothing. I see motorbike riders rip these parks up and ring authorities up and nothing ever gets done about it. If you go over to Karawatha Forest you'll see all the water bottles, the food packaging walkers throw everywhere, they cut the fences around the quarry every week to go swimming and council goes back and repairs it every second week. The fire roads in there are causing more erosion than any MTB trail in there, council has gone in there and cut down some green trees to block trails that the green groups have complained about by making allegations that mountain bikers swiped a school walking group.
I personally don't think it's about conservation anymore, especially when they state to go build trails out of the Brisbane area, these parks have been mined, logged many times over and been used as dumping grounds, mining excrement left all over the place. Why the fuck would you go and ruin virgin land out of the Brisbane area, like the conservation act doesn't matter to them as long as the trails are away from them.