No there wasn't any build up on the radiator that I could see. I took it (the radiator) to work and blew it out with air. I also capped off the inlet and the opening to the reservoir and blew it out that way as well (100 psi). No debris at all and good air flow. It sounds of though you're thinking the water pump (what else could it be at this point).
I don't see what could be wrong with it, its not leaking, it doesn't wobble, and its only an impeller. Any clues as to what would fail?

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1986 300sdl
1984 300sd
1988 300se
1973 vw thing