I'm seeing spots. They look like dust or worse on the sensor. However, I noticed that they appear much more pronounced and visible the lower my shutter speed. In the photos below, the first was taken at 1/25" and the second one at 1/400". If I zoom to 1:1 on my 29" monitor I can still see faint spots on the second image in the upper left corner but they don't look anything like the spots that are all over the first photo.
Anyone seen this before? Do I just need to clean my sensor again? What could cause these spots to be more or less visible depending on shutter speed?