It's trying to tell you you're doing something wrong and please don't kill yourself by CO poisoning. The carbon monoxide danger is pretty serious, would probably be a liability issue these days without a tailgate open alarm.

I don't know of any way to disable the beeper unless you can find a fuse that will kill that circuit - and I'm not sure it won't kill a lot of other stuff.

I agree the Rogue isn't really designed for this, but for heaven's sake, hauling a few pieces of 8' lumber shouldn't be a problem.

Failing everything else, how about earplugs?

For long boards and other similar stuff (PVC pipe, etc.) I used the cross bars on my last SUV to bungee/tie down the load to bring it home from the DIY. Bought cross-bars for the Rogue for the same reason.