Potterton 100e Boiler - making unusual purring sounds?

I have recently noticed that the boiler has begun making a purring or whirring noise.

This happens when the heating has been on for a while, or I am running a lot of hot water. If I stop running the tap, the boiler will continue to make this noise for a minute of so before settling down.

Besides this strange noise, the boiler appears to work okay.
Final note- The water pressure metre is currently around the "1" mark. I will look to top this up to "2" soon.