Brown blotches on frames then vanish

[footman]

Among the hundreds of shots I took at an airshow yesterday there suddenly appeared brown blotches like flecks of dried mud.
These were present for about 20 frames and then vanished.

I'd be grateful for any ideas.

 

[cosmicma]

on the photo you've posted is it not crud blowing about because of the helicopter rotor blades ?

a bit like this one

 

[footman]

Erm, yes I think you've hit the nail firmly on the head, thanks