Starting problem '96 suburban

I have a 96 suburbban with a 350 vortec in it. Just recently it started giving problems starting. i.e. Starter would only click once when key was turned, the lights and horn were bright and loud. Charged battery anyway to full. Same click. When this happens the battery drains.
had amechanic check it out " he said starter was fine". long story short I can unhook the positive cable for a moment and the truck will start. could this be a relay sticking and if so which one?
Thanks, Todd