1999 GMC Yukon turn signals

My turn signals, brake lights, and 4-ways all work fine, however the blinking sound that the flasher unit makes is constant. If you touch the brakes pedal enough to ignite the brake lights, it stops. I traced the sound to the flasher unit under the dash and replaced it, however this made no difference. The shop wants almost $500 to replace the turn signal switch. Is this what it is?