1997 Chevy 1/2 Ton Won't Start

Hello All,
I have a 1997 Chevy 1/2 Ton, 4x4, 350 with roughly 145,000 miles on it. When I try to start it, it will not turn over. It acts like the battery is dead. To provide some more info when I first get in the truck and turn the key to the first position (accessory) the steady tone that you normally hear before you start sounds good nice and strong as soon as I attempt to start the truck (turn the key to full start) nothing happens and the original tone now sounds week like a dead battery. Other functions in the truck also act like the battery is weak. The auto door locks operate slower etc. To give some more background about a week ago this first started. I noticed my positive lead was heavily corroded. I thoroughly cleaned it off and the truck seemed to fire right up. I took the battery in and it tested good. The starter tested good. I think it is odd that when the key is first turned to accessory the truck seems fine the fuel pump primes and everything seems fine as soon as the key is fully engaged that’s when the issue starts. There are also some odd noises going on under the hood when this happens, some "clicking" noises. It doesn't seem to be coming from the starter. Is there a short somewhere? Is this a common problem on this year of Chevy? Any ideas or guidance would be greatly appreciated.