I have a '94 GMC 2500K pickup w/ a 6.5 Litre turbo diesel engine. When I turn on the ignition switch READY TO START the engine a clacking sound begins in the area of the passenger’s feet and continues for 20 seconds, then stops. The clacking does not occur while driving but re-occurs when I turn the ignition switch to the OFF position. It does not occur when the switch is in the accessory position. The only thing that I know that is wrong with the truck is that it does not have the belt-driven vacuum pump that is normally mounted on the front of the engine.
Does the heating and AC system work OK? I think the noise may be from the HVAC system, perhaps trying to move one of the flaps in the system. Many automotive HVAC systems are operated, at least partially, by vacuum, and the lack of the vacuum pump could have something to do with the noise.
Wild guess, perhaps, but it’s all I’ve got.