2000 Silverado turn signal

My Silverado recently developed a left turn signal problem... the left turn signal would work but it would make the odometer in the cluster blink as it
was blinking....
Now the left hand turn signal does not work and the (left outside) turn signal lights are on (only the left side) and I can't get them off. When I try and put the signal light lever in the left direction I hear a humming noise.
Could this mean a bad flasher? Are the flashers the small boxes I see on the floor of my truck? (near the firewall) There are two small grey boxes I am assuming there is one box for each flasher are these boxe interchangeable?
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