The fan in the power supply is making a lot of noise and I need to remove the power supply and inspect it. The wires run under the heat sink, do I need to remove the heat sink to get the wires out?
Dell service manuals, which have full disassembly instructions, are available online online at support.dell.com. Here's the Precision 390 one: ftp://ftp.dell.com/Manuals/all-products/esuprt_desktop/esuprt_dell_precision_workstation/precision-390_user%27s%20guide_en-us.pdf
Unfortunately, the only relevant instruction appears to be:
Disconnect the DC power cables from the system board and the drives.
Note the routing of the DC power cables underneath the tabs in the computer frame as you release the tabs and remove the cables from the system board and drives. You must route these cables properly when you replace them to prevent their being pinched or crimped.
If it does turn out that you have to remove the heat sink to get the cables out - page 96 of the service manual - make sure to clean off and replace the thermal paste between the heat sink and processor. (general thermal paste instructions)