Get plumber back as first port of call. Did he supply as well as fit? If he supplied then he has an obligation to come back and not just try and pass the buck direct to the manufacturer.
In any event he should be able to diagnose the problem without waiting for a manufacturers engineer even if that means "send a new one Mr Triton under warranty and I'll bill you for fitting it".
On a more practical level though please quantify "excessive". You may be able to live with it if its not very much. If it is a lot then you can ask your plumber to isolate it at the full bore isolating valve he fitted if there wasn't one in situ before.
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