shower diverter making noise

I have noticed recently that when I switch my shower diverter from shower to tub I am hearing a 'thump'. The water is not running when I do this.
Is this just a bad diverter that needs replacing? I haven't noticed any water on floor.
This is on inside wall and it is shower-tub combo. It is pretty old, possibly 20 years. Make is PriceFister.
Thanks for any help.
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