Hmmm, so either the 50mm has some serious internal reflections or the last lens on the inside is not coated well enough so that it reflects the light that bounces back from the sensor (a well-known prob with DSLRs).
You can test this by dismounting the 50mm and look at the back-lens critically: does it reflect more than the front-lens? Compare it to the back-lens of your 18-135mm. If the 50mm "shines" on the backside, that is the most probable problem and is not going away.
If the backsides of both lenses are comparable (coating-wise), the reflection should be internal. In that case stopping down might reduce the prob. But then, what is the charm of a fast F1.8 lens?!
Again, you can see that contrast is more important than noise
in modern DSLRs. See my rant here
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