Why Does My Budgie Make Odd Noises?

My year and a half old male budgie, Jasper, has of late decided to become VERY protective of his mate and any time I come near the cage he makes a low tweeting, gurgling huming sort of noise. The strange thing is none of the other males make this weird noise-what is he trying to tell me? and is it his special noise he made himself?

Thanks, Luce