My 2013 SL550 was recently in the shop to fix a pervasive background squeaking noise that had appeared during my 3 months of ownership and approx 1000 miles of driving. Fortunately the dealership was able to reproduce, acknowledge, and fix the problem. I'm reporting it here in case any other SL owners experience the same issue.
The squeaking occurred at fairly low speeds, e.g. 5-10 mph, but not directly related to road roughness. I only heard the noise with the top closed and it seemed to come somewhere in the rear and on the driver's side. I recorded a short 15-sec clip for my service adviser (you have to turn up the volume to hear it clearly, at the very end of the clip I turn on the turn signal so you can use that to gauge how loud the squeaking was):
The reported fix on my service paperwork was:
road test verified concern heard ticking noise from roof area with roof up. with roof down no noise. inspect roof found roof has a lot of sand on the seals. sand and dust might cause squeaks. requested detail then clean out all dust and sand. road test after detail cleaned out all the seal roof seals. found clicking noise at front roof latches. inspect latches ok lubed striker piece. road test no more noise heard this time.