1996 A6 2.5TDi noise

Hi Chaps,
My '96 A6 2.5Tdi makes a slight sqeak/sqeal noise very briefly at startup - just as the engine catches, not whilst cranking, and again
just as the engine is switched off. It sounds sort of like a slipping belt, but if that were the case I'd expect it to slip when the engine was running.
I don't think it's too important - engine runs perfectly - but I'm curious as to what it might be. Any suggestions?
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