Noisy Boiler/Tank

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I recently moved into a new house. In the last couple of weeks the I've had the heating on every day for for 5-6 hours. After a couple of hours, there is a drilling like noise coming from either the hot water tank or the boiler. They are right next to each other so i dont know which.It only lasts a second or two but will repeat for about a minute. After 10 mins it will stop. But the come back later. Any thoughts??