Clicking sound from the dash from behind the blower motor in the vent on 94 GMC Sierra

I see this problem is listed as a 2000 GMC 1500, yet you discribe the problem as on a 1994 GMC. The two trucks are very different, if it's a '94, them there are relays mounted on the passengers side firewall under a plastic shield, these will make clicking noises. If it's a 2000, the most common cause for that type of noise is the heat shields on the exhaust manifolds . If the noise is only present after [and sometimes while ] the motor is running, that's the most likley the problem.