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Question: Throttle Body Issue

I have a 1999 A4 2.8L with a throttle body problem. It's been in the shop and was being drive to get the readiness code so it would pass inspection when a new CEL came up and it went into a sort of limp mode. There is a substantial, at least 50%, loss in power. Holding the RPMs to anything above 1,100 the issue is noticeable--the RPMS hold where they should, drop to near nothing and come back up on their own. After getting it home (needed the car) I disconnected the battery and let the ECU plugs drain. I attempted to realign the throttle body by turning the full power back on (engine off) and waiting for the whine at the timing belt. Instead I heard a quick clank. Could it be the throttle position sensor? Would adding a Heet octane booster/water remover, or replacing the fuel filter help at all? The mechanic, who specialize in Audis, BMWs, and Mercs ran out of time to diagnose after about 12 hours (I needed the car). Any ideas to save him time and me money at the garage would be extremely and unimaginably appreciated. Thanks