Rattle (buzz) at 1200rpm?

[Jalfrezi]

Lately I've been driving my 3.0 more sedately. However in higher gears I get a brief buzz or rattle around 1200 rpm. Sounds a bit like a spring rattling up against something.

Any ideas? Getting quite annoying now, but more importantly would like to fix it before it gets worse

 

[mmm-five]

It might be the cat heat shield 'tizzing' at a frequency that happens to resonate at lowish revs.