iMac Weird Boot Up Pattern

I haven't noticed this before and wonder if I should be concerned. It seemed to start after I used Bootcamp, installed Windows 7, changed my mind and removed it again. Then reinstalled OSX (for other non-related issues). It seems to be a 'streaky' kind of 'marble' pattern during the boot process which goes away when it's all fired up. Any ideas?


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...actually it's there all the time. Just need the right level of brightness to see it.

 

Many thanks...

...will investigate further though if that's all it is and won't effect functionality *and* I find it hard to see routinely anyway, maybe I'll ignore! Response appreciated