You will find lots of dust, and quite probably sticky gooey residue around this part of the PCB. This is heat-sealant for the thermal pads that lie on the row of R35 regulators. The heat sealant often leaks from the heatsink and drips onto the PCB. The gooey residue captures dust and should be cleaned away. Use cotton buds and isopropyl alcohol.

Clean both sides of the whole PCB between the components with a cotton bud dipped in isopropanol. Take care that cotton threads are not caught up on electronic components and remain behind.

In this photograph the tops of the black plastic stays holding the northbridge heatsink in place are visible.