Noisy heating and hot water system

[Peachy1965]

We have an old vented heating and hot water system, run by an oil boiler. We have loud banging noises when on and since having a new pump it’s even worse. It sounds as if air is being sucked in and when you hold the cold and vent pipe in the airing cupboard you can feel it vibrating. I’m pretty convinced it’s pulling in air. Is this pipe work the cause? The feed is only 15mm although vent is 22 and it’s joined together in the airing cupboard. I’ve attached some pictures. Thanks. We’ve tried several engineers and nobody seems interested in fixing it, but it’s unusable because of the noise. Thanks.