BAC condenser fan with axial movement, looks like the white U channel pillow block support is flexing, the top of the block exaggerates the movement but the block seems to be fixed securly to the support.

Think it was 35Hz,
https://www.flickr.com/photos/s34n/5...n/photostream/

About 15Hz,
https://www.flickr.com/photos/s34n/5...n/photostream/