2004 TDCi metallic sound when starting

2004 Focus TDCi metallic clunking sound when starting
Every time the engine is started there is a metallic clunking sound. Once the engine starts and I release the ignition key the
metallic sound stops and the engine runs fine. I thought the sound could be coming from the starter motor but standing facing the engine from the front, the noise seems to be coming from the left. I believe the starter motor is at the right.
The car runs very well with no other problem apart from the metallic clunking sound when starting.
Any ideas what the problem might be?
Thanks
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The Focus went to two repair places this morning.
First one confirmed that the noise was to the left not flywheel end and thought it might be a problem with an engine mounting.
Second place took off the bottom splashguard cover and confirmed the problem to be the auxiliary drive belt tensioner.
A pleasing outcome.
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