Tried supercheap/autobahn they were useless. Got the covers on ebay fit perfect. They are awesome 10min install.
http://www.ebay.com.au/itm/FRONT-Seat-Covers-Nissan-Navara-NP300-Premium-Neoprene-Waterproof-100-Fit-/252139657751?epid=19003886120&hash=item3ab4b1da17:g:VYkAAOSwA4dWK3ew
corner alert
The noise could be a sharp corner alert noise. Stumped me for a while rang Nissan no help, found it in navigation settings. Like a warning beep like you described that a sharp turn is approaching.