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Just to add, I had exactly this problem with a Opel Manta 1.8 back in the 1980s
This too had a manual ZF box and chattered quite badly when hot
I tried to get it replaced but was told it wasnt a fault and even had a letter from GM tech support department to say it was a characteristic
I guess consumer law is somewhat better these days though so you still might get some goodwill. Certainly no harm in following it up
Otherwise an oil change would be a good place to start
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