PCB ground plane noise
Step 1: Scrap your existing design.

Step 2: Read the TI data sheet for the TL494. Especially read through the "Layout" section near the end. There is a PCB layout example in that section though it is only a partial design. Here is TI's page with the full datasheet and application notes: http://www.ti.com/product/TL494

Step 3: Start a new schematic. Draw the schematic with the datasheet layout information in mind. Obviously the schematic does not need to define the final PCB layout but try to draw it as if it did. When done in this way later PCB design decisions such as component placement and trace positioning can be guided by the way you drew the schematic. As in the datasheet layout suggestions keep high current lines short, thick, and away from sensitive low current control/feedback lines. 

Breaking the ground plane up sometimes helps, but it can also complicate some of the design. Consider making the ground plane with only a partial divider but still keep a wide connection between each side. The high current components and traces will stay on one side and the low current sensitive parts on the other. Be sure to place decoupling caps where the datasheet recommends. 

As a final thought, note the pin arrangement of the chip itself, certain function pins are grouped to one side (for e.g. 1 thru 6, and 15, 16), this is done purposely and gives you additional hints to the suggested physical layout.

For even more help here is an older StackExchange Q/A that went into some complex details of a TL494 design. The design is relatively close to yours and a PCB layout is included. Buck converter, squeaking/whining inductor