Dell Optiplex 5055 with new CPU Fan.

Although it is AM4 socket, Dell is lazy enough not to design a proper AM4 oem cooler for it. So the motherboard actually uses intel socket heatsink mounting hole spacing (square 75x75 mm).  Dell does the same thing in Aurora Ryzen edition: amd cpu socket, but hybrid intel heatsink spacing (LGA1151).  Why?  It is simply not worth Dell effort to invest in a proper oem am4 heatsink because Dell makes so fewer amd pc compared to vast number of intel pc.

5055 uses same pancake heatsink assembly as 5050 which is intel based.  

having said that, you can just try to replace your oem cpu fan w a more quiet 80x80 mm non-Dell fan while keeping the heatsink (NB: many non-Dell fan may throw a bios startup error), or you can replace the whole cooler w a low profile one (you have 5055 SFF) but bear in mind Dell heatsink uses M3 thread backplate built in chassis floor.  New heatsink needs to be c/w M3 backplate too.

PS AM4 heatsink spacing is rectangular.  The AM4 socket specifies the 4 holes for fastening the heatsink to the motherboard to be placed in the corners of a rectangle with a lateral length of 54×90 mm.

The 4 holes for fastening the heatsink to the motherboard are placed in a square with a lateral length of 75 mm for Intel's sockets LGA 1156, LGA 1155, LGA 1150, LGA 1151 and LGA 1200