This is on my 600D. I'm a noob so forgive if it's obvious. There's a clear moire-like interference pattern on the viewfinder on the 50mm which is not as obvious at all on the kit lens. It's especially obvious when looking at out-of-focus bokeh but doesn' show up in the photos.
It sounds like you're observing the laser etched glass focussing screen. A focusing screen requires a rough surface so an image can be formed for you to view through the viewfinder.
Ground glass used to be used for this purpose but now glass etched in a circular pattern with a laser is used, to improve the brightness of the viewfinder when used with slower lenses. This pattern is probably what you're interpreting as moire.
Here's what a standard DSLR focussing screen looks like close up: