It only knocks once when I am reversing on full lock.......Please help

We had this problem with an 1988 hiace we had in New Zealand, thought it was really serious but took it to a garage- all it was just a restrictor that stops your wheel turning too much and hitting your wheel arch, it's an arm of metal that hits a socket when you are on full lock, it just needed a good squirt of grease back into the socket to make the noise go away- took less than 5 minutes to do.

Powervanman is correct. They are limit stops on the front steering legs. If you get under there and have a look you will
see the stops, and where they rub on the front crossmember off memory when you turn full lock.

When new from the factory, I think the knuckle has a rubber boot on it which stops the metal on metal, but this eventually wears
down and the noise you hear is the stud griding against the stop.

A good slop of grease on the knuckle will stop this.