Constant spluttering

Hi. I've been trying to solve an issue for a dental practice with their water. It splutters as if there is air being pushed out on both hot and cold. It's a combi system (all mains pressure). This issue has been going on for two years and began when work was being conducted over the road. The problem is worst at the start of the week and almost stops by Friday. Monday though it's back with a vengance. There are no local ongoing works. The plate heat exchanger is new as Worcester recommended that could be the issue, with boiler water leaching back into the cold main. To me it seems it MUST be an issue with the incoming water but the water board have been out and checked it and said it's ok and runs fine and clear with no splutters. Any ideas?
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