Do I need a plumber???

Hello,
We live in a dual occupancy block (rear of 2 houses) and our pipes vibrate at about the same time of night without any of our taps being turned on. Occasionally when we turn the shower on we also get a knocking noise that diminishes quickly. Over the last few days we have been getting a 'groaning' noise that I assume is related to the same porblem.
Both houses have separate meters so I'm not sure that there could be some connection between the front house's taps being turned on and the noises in our house. Other than the shower issue no taps have been on when the knocking or groaning accur. Are there any tests that I could do before getting a plumber to look at it? Is it a problem with out water pressure? Any ideas? Or is our house haunted????? Advice appreciated.
Cheerio,

Mark


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