Challenge: Finding "Echo in letters" problem.

Anyone know what may be the cause of this strange effect?

It isn't resonation because it occurs when printing straight in outer contours (No infill, the machine is not shaking),it occurs always just after print a hole like letters or stamped figures.
Perhaps a marlin spline bad aproximation?...

It looks like a mechanical problem, the ripples comes from vibration which is triggered when the printer navigates a sharp corner.

If the printer isn't vibrating, your belts is probably too loose.

Rippling, Vibration and Resonance [wiki.arcol.hu]
Trouble shooting print quality [www.reprap.org]
High Speed Printing [forums.reprap.org]
Printing small objects [richrap.blogspot.co.uk]
Polyholes [hydraraptor.blogspot.com]
Interrupted print, how to restart. [forums.reprap.org]