As you move through the case, you'll get a second "eerie tone" call. It happens at some point after you've been to the safe house and the Cathedral hub.
The voice asked you to cut the power to the trash compactor at the train station.
Where is the trash compactor?
You need to cut the power literally. To do that, you'll need some sort of cutting tool.
The pruning shears that were next to the mailbox in Giffnock should do nicely.
Use those shears on the large red cable to the left of the trash compactor, and you're done.